@Asifahmed You can only customize a certificate using variables if you're going to have it generated via a Configuration Profile (e.g. a Certificate payload that request a cert via ADCS Connector or Jamf PKI Proxy, or a SCEP payload to request a certificate via SCEP). If you have a .cer file you have already created the certificate and it cannot be modified.

So for ADCS and SCEP how can i do it? In CP where i need to put the variable, can you please give a screenshot so that i can understand easily.

This is pretty basic stuff. All of the information on using variables in a Configuration Profile can be found in Jamf's documentation.

Start with this one: https://learn.jamf.com/bundle/jamf-pro-documentation-current/page/Computer_Configuration_Profiles.html

The following Jamf article discusses creating a certificate payload for 802.1x authentication: https://docs.jamf.com/technical-papers/jamf-pro/8021x/10.0.0/Distributing_802.1X_Settings_to_Computers.html

Do you already have ADCS or SCEP setup in Jamf? Because you have to have one of those in place first before you can use it in a profile. We're using ADCS, so I can't speak to how it's done in SCEP.