Converting cer or pem file to p12 (ERROR: Could not read private key from -inkey file)

Question

I bought a (E-mail ID Business (S/MIME)) certificate from certum, hoping to use to sign pdf files more affordable. They have send me the files in plain(pem) and in binary(cer).
Here is the file list I downloaded:

• Certificate chain Certum Digital Identification CA SHA2.cer
• Certificate chain Certum Digital Identification CA SHA2.pem
• Certificate chain Certum Trusted Network CA.cer
• Certificate chain Certum Trusted Network CA.pem
• Certificate.cer
• Certificate.pem

I tried to use below command to create p12 file.

"openssl.exe" pkcs12 -export -in D:\xampp_data\MIME\Certificate.cer -inkey D:\xampp_data\MIME\Certificate.cer -out Certificate.p12 -name "MyCert" -password pass:MyCert

• When using the command with D:\xampp\php\extras\openssl\openssl.exe, windows 11 console does not gives any error. And there is no output file too.
• When using the command with C:\Program Files\OpenSSL-Win64\bin\openssl.exe, windows 11 console gives below error. Win64 OpenSSL v3.0.5 Light

D:\xampp_data\MIME>"C:\Program Files\OpenSSL-Win64\bin\openssl.exe" pkcs12 -export -in D:\xampp_data\MIME\Certificate.cer -out Certificate.p12 -name "MyCert" -password pass:MyCert
Could not read private key from -in file from D:\xampp_data\MIME\Certificate.cer