My iPhone 13 doesn’t have the option to add eSIM?

What is the model number under Settings ➜ General ➜ About ➜ Model.

Provide both or at least the longer one that does not start with an A.

That is a Japanese model. and should offer the option to add en eSim in Settings ➜ Cellular / Mobile.

Can you provide a screen shot of that page in the Settings App?

Very strange.

Your carrier may be blocking it somehow

Do you see a secondary IMEI number in Settings ➜ General ➜ About?

Do you have a corporate account or is the iPhone managed by your employer? They can remove the eSim option via a corporate MDM profile.

If neither of the above applies, then there may be an issue with the iOS software and you may need to restore the iPhone to factory settings and set it up again and see if the option is available.

It is the last remaining option before having to set up an appointment at a genius bar to have it checked out.

This a user to user forum; not Apple directly.

Please be patient.