vim-airline doesn't show git branch

Question

I tried to use vim-airline, but the git branch of the current directory doesn't show up on the screen. Link to Screen Image

I'm currently using Neovim 0.7.2 in a mac, and my vimrc is as shown below.

:echo fugitive#head() returns master, but :echo airline#extensions#branch#head() returns nothing.

I tried let g:airline#extensions#branch#enabled = 1, still doesn't work.

Thank you in advance :)

call plug#begin()
    Plug 'preservim/nerdtree'
    Plug 'preservim/nerdcommenter'
    Plug 'vim-airline/vim-airline'
    Plug 'vim-airline/vim-airline-themes'
    Plug 'sheerun/vim-polyglot'
    Plug 'neoclide/coc.nvim', { 'branch': 'release' }
    Plug 'morhetz/gruvbox'
    Plug 'vimpostor/vim-lumen'
    Plug 'ryanoasis/vim-devicons'
    Plug 'tpope/vim-fugitive', { 'tag': 'v2.3' }
call plug#end()
set nu 
set hlsearch
set autoindent 
set cindent
set ts=4
set sts=4
set shiftwidth=4
set laststatus=2 
set showmatch
set ruler
set mouse=a
set guifont=DroidSansMono_Nerd_Font:h11
let g:airline_powerline_fonts = 1
if has("syntax")
    "syntax on
endif
colorscheme gruvbox

"set termguicolors
"sync colorscheme to system preference
let g:lumen_startup_overwrite = 1
autocmd User LumenDark set background=dark
autocmd User LumenLight set background=light


nnoremap <leader>n :NERDTreeFocus<CR>
nnoremap <C-n> :NERDTreeToggle<CR>
nnoremap <C-f> :NERDTreeFind<CR>
" enable NerdTree at startup, then get return to original document
autocmd VimEnter * NERDTree | wincmd p

" disable line number in terminal
autocmd TermOpen * setlocal nonumber norelativenumber

" (\r) to refresh NerdTree
nmap <Leader>r :NERDTreeFocus<cr>R<c-w><c-p>

" lets us escape nvim terminal
augroup TerminalBehavior
  autocmd!
  autocmd TermOpen * startinsert
  autocmd WinEnter term://* startinsert
augroup ENDheader

" moving between buffer remap
tnoremap <C-h> <C-\><C-n><C-w>h
tnoremap <C-j> <C-\><C-n><C-w>j
tnoremap <C-k> <C-\><C-n><C-w>k
tnoremap <C-l> <C-\><C-n><C-w>l

noremap <C-h> <C-w>h
noremap <C-j> <C-w>j
noremap <C-k> <C-w>k
noremap <C-l> <C-w>l

" autocomplete braces
inoremap { {}<Esc>ha
inoremap ( ()<Esc>ha
inoremap [ []<Esc>ha
inoremap " ""<Esc>ha
inoremap ' ''<Esc>ha
inoremap ` ``<Esc>ha

" Use tab for trigger completion with characters ahead and navigate.
" NOTE: Use command ':verbose imap <tab>' to make sure tab is not mapped by
" other plugin before putting this into your config.
inoremap <silent><expr> <TAB>
      \ pumvisible() ? "\<C-n>" :
      \ CheckBackspace() ? "\<TAB>" :
      \ coc#refresh()
inoremap <expr><S-TAB> pumvisible() ? "\<C-p>" : "\<C-h>"

function! CheckBackspace() abort
  let col = col('.') - 1
  return !col || getline('.')[col - 1]  =~# '\s'
endfunction

" Use <c-space> to trigger completion.
if has('nvim')
  inoremap <silent><expr> <c-space> coc#refresh()
else
  inoremap <silent><expr> <c-@> coc#refresh()
endif