SSL CSR generation for external certificate renewal

Hi Team, Currently we are using F5 APM 11.5.3, I was trying to renew my external certificate, while Generating CSR under the tab Subject Alternative Name have give the name which have mentioned in common name but its thrown error( An error occurred while trying to process your request).

 

Is it mandatory to give the Subject Alternative Name, Im generating external certificate so having little doubt. I tried generate the CSR without the Subject Alternative Name, im able to generate the CSR successfully. Need your inputs on Subject Alternative Name.....Plz