eSignature can be used for all current and prior year returns but your credits expire automatically on 12/31 of each year.  See "When will eSignature credits expire?" in the following article.

You should be able to use the credits purchased for TY2020.  In fact, it can even be used for signing other documents such as EL.  The reason you can't use the credit now is because eSignature is not available in the early releases.  See "When will eSignature be available in my program?" in the following article for more info.

https://proconnect.intuit.com/community/manage-integrations/help/intuit-esignature-faqs-resource-gui...