MPI fortran 90 - Abort errors on nodes during execution

Question

I am trying to execute this program using MPI that I have written in fortran:

Program CartesianGrid2D
    Implicit None
    Include 'mpif.h'

    !----------------------------------------!
    ! Setting of the computational grid      !
    Integer, Parameter :: NDIM = 2 ! number of space dimensions
    Integer, Parameter :: IMAX = 200 ! number of grid points in x-direction
    Integer, Parameter :: JMAX = 200 ! number of grid points in y-direction
    Real, Parameter :: x0=0.0 !min x-coord
    Real, Parameter :: x1=1.0 !max x-coord
    Real, Parameter :: y0=0.0 !min y-coord
    Real, Parameter :: y1=1.0 !max y-coord
    !----------------------------------------!
    ! Local variable declaration
    TYPE tMPI
        Integer :: myrank
        Integer :: nCPU
        Integer :: status(MPI_STATUS_SIZE)
        Integer :: iStart, iEnd !idx of starting and ending cell in x-dir
        Integer :: jStart, jEnd !idx of starting and ending cell in y-dir
        Integer :: imax, jmax !number of cells within each rank
        Integer, Allocatable :: mycoords(:) !point coords of the subgrid
        Integer :: iErr !flag for errors in x-dir
        Integer :: x_thread !number of CPUs in x-dir
        Integer :: y_thread !number of CPUs in y-dir
    End TYPE tMPI
    TYPE(tMPI) :: MPI

    Logical, Allocatable :: periods(:)
    Integer, Allocatable :: dims(:)
    Integer :: TCPU, BCPU, RCPU, LCPU !neighbor ranks of myrank
    Integer :: i, j, idx, jdx, source
    Integer :: COMM_CART !cartesian MPI communicator
    Real :: dx, dy
    Real, Allocatable :: x(:), y(:) ! grid coordinates
    !----------------------------------------!

    ! 1)  MPI initialization

    CALL MPI_INIT(MPI%iErr)
    CALL MPI_COMM_RANK(MPI_COMM_WORLD, MPI%myrank, MPI%iErr)
    CALL MPI_COMM_SIZE(MPI_COMM_WORLD,MPI%nCPU,MPI%iErr)
    ! check the number of CPUs
    If(MOD(MPI%nCPU,2).ne.0) then
        print *, 'ERROR. Number of CPU must be even!'
        CALL MPI_FINALIZE(MPI%iErr)
        Stop
    End if

    CALL MPI_BARRIER(MPI_COMM_WORLD,MPI%iErr)

    ! 2) Create a Cartesian topology

    ! check the number of cells
    
    If(MOD(IMAX,2).ne.0) then
        print *, 'ERROR. Number of x-cells must be even!'
        CALL MPI_FINALIZE(MPI%iErr)
        Stop
    End if

    If(MOD(JMAX,2).ne.0) then
        print *, 'ERROR. Number of y-cells must be even!'
        CALL MPI_FINALIZE(MPI%iErr)
        Stop
    End if

    ! Domain decomposition
    MPI%x_thread = MPI%nCPU/2
    MPI%y_thread = MPI%nCPU - MPI%x_thread

    Allocate(dims(NDIM), periods(NDIM), MPI%mycoords(NDIM))

    dims = (/ MPI%x_thread, MPI%y_thread /)
    periods = .FALSE.

    CALL MPI_CART_CREATE(MPI_COMM_WORLD,NDIM,dims,periods,.TRUE.,COMM_CART,MPI%iErr)
    CALL MPI_COMM_RANK(MPI_COMM_WORLD, MPI%myrank, MPI%iErr)

    ! 2.3) Find CPU neighbords
    CALL MPI_CART_SHIFT(COMM_CART,0,1,source,RCPU,MPI%iErr)
    CALL MPI_CART_SHIFT(COMM_CART,0,-1,source,LCPU,MPI%iErr)
    CALL MPI_CART_SHIFT(COMM_CART,1,0,source,TCPU,MPI%iErr)
    CALL MPI_CART_SHIFT(COMM_CART,-1,0,source,BCPU,MPI%iErr)

    ! coordinates of the subgrid
    CALL MPI_CART_COORDS(COMM_CART,MPI%myrank,NDIM,MPI%mycoords,MPI%iErr)
    MPI%imax = IMAX/MPI%x_thread
    MPI%jmax = JMAX/MPI%y_thread
    MPI%iStart = 1 + MPI%mycoords(1)*MPI%imax
    MPI%iEnd = MPI%iStart + MPI%imax - 1
    MPI%jStart = 1 + MPI%mycoords(2)*MPI%jmax
    MPI%jEnd = MPI%jStart + MPI%jmax - 1


    ! 3) Comoute the real mesh
    dx = (x1-x0)/Real(IMAX-1)
    dy = (y1-y0)/Real(JMAX-1)
    Allocate(x(MPI%IMAX))
    Allocate(y(MPI%JMAX))

    idx  = 0
    Do i = MPI%iStart, MPI%iEnd
        idx  = idx + 1
        x(idx) = (x0-dx/2.) + (i-1)*dx
    End do

    jdx  = 0
    Do i = MPI%jStart, MPI%jEnd
        jdx  = jdx + 1
        y(jdx) = (y0-dy/2.) + (j-1)*dy
    End do

    ! 4) Plot the output and finalize the program

    CALL ASCII_Output(x,y,MPI%imax,MPI%jmax,MPI%myrank)
    CALL MPI_FINALIZE(MPI%iErr)


End program CartesianGrid2D


Subroutine ASCII_Output(x,y,imax,jmax,myrank)
    !-----------------------------------------!
    Implicit None
    !-----------------------------------------!
    Integer :: imax, jmax, myrank
    Real :: x(imax), y(jmax)

    Integer :: i, j, DataUnit
    Character(len=10) :: cmyrank
    Character(len=200) :: IOFileName

    !-----------------------------------------!

    Write(cmyrank,'(I4.4)') myrank
    IOFileName = 'CartesianGrid_output-'//TRIM(cmyrank)//'.dat'
    DataUnit = 100+myrank
    Open(Unit=DataUnit, File=Trim(IOFileName), Status='Unknown', Action='Write')

    Write(DataUnit,*) imax
    Write(DataUnit,*) jmax
    Do i = 1, imax
        Write(DataUnit,*) x(i)
    End do
    Do j = 1, jmax
        Write(DataUnit,*) y(j)
    End do

    Close(DataUnit)


End Subroutine ASCII_Output

However whenever I try to execute I have got this list of errors popping up:

Abort(795947788) on node 0 (rank 0 in comm 0): Fatal error in internal_Cart_create: Invalid argument, error stack:
internal_Cart_create(102): MPI_Cart_create(MPI_COMM_WORLD, ndims=2, dims=0x60000384c9f0, periods=0x60000384c9e0, reorder=1, comm_cart=0x16f8532a0) failed
MPIR_Cart_create_impl(43): Size of the communicator (6) is smaller than the size of the Cartesian topology (9)
Abort(863056652) on node 1 (rank 1 in comm 0): Fatal error in internal_Cart_create: Invalid argument, error stack:
internal_Cart_create(102): MPI_Cart_create(MPI_COMM_WORLD, ndims=2, dims=0x600003030760, periods=0x600003030860, reorder=1, comm_cart=0x16ef332a0) failed
MPIR_Cart_create_impl(43): Size of the communicator (6) is smaller than the size of the Cartesian topology (9)
Abort(460403468) on node 2 (rank 2 in comm 0): Fatal error in internal_Cart_create: Invalid argument, error stack:
internal_Cart_create(102): MPI_Cart_create(MPI_COMM_WORLD, ndims=2, dims=0x6000039647d0, periods=0x6000039647e0, reorder=1, comm_cart=0x16d05f2a0) failed
MPIR_Cart_create_impl(43): Size of the communicator (6) is smaller than the size of the Cartesian topology (9)
Abort(191968012) on node 3 (rank 3 in comm 0): Fatal error in internal_Cart_create: Invalid argument, error stack:
internal_Cart_create(102): MPI_Cart_create(MPI_COMM_WORLD, ndims=2, dims=0x600000b78470, periods=0x600000b784e0, reorder=1, comm_cart=0x16f75f2a0) failed
MPIR_Cart_create_impl(43): Size of the communicator (6) is smaller than the size of the Cartesian topology (9)
Abort(997274380) on node 4 (rank 4 in comm 0): Fatal error in internal_Cart_create: Invalid argument, error stack:
internal_Cart_create(102): MPI_Cart_create(MPI_COMM_WORLD, ndims=2, dims=0x6000039843e0, periods=0x6000039843d0, reorder=1, comm_cart=0x16ba532a0) failed
MPIR_Cart_create_impl(43): Size of the communicator (6) is smaller than the size of the Cartesian topology (9)
Abort(259076876) on node 5 (rank 5 in comm 0): Fatal error in internal_Cart_create: Invalid argument, error stack:
internal_Cart_create(102): MPI_Cart_create(MPI_COMM_WORLD, ndims=2, dims=0x6000016cab90, periods=0x6000016caaa0, reorder=1, comm_cart=0x16d3232a0) failed
MPIR_Cart_create_impl(43): Size of the communicator (6) is smaller than the size of the Cartesian topology (9)

What I first do is : mpif90 -cpp -lmpi NameOfTheProgram.f90 and then whenever I execute the a.out I do mpirun -np 6 ./a.out

Running this on a MacBook Air M1 (Whenever I have to run fortran I usually use a gfortran compiler).

MPI fortran 90 - Abort errors on nodes during execution

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