There are a few reasons this might happen:
1. You are using a different email address than the one you purchased your licened with. On the Stripe checkout page please use the email your want to assign the license for. If you purchased your license with a different email please contact us.
2. You used an email from another email provider than Google. Our licenses can be used only with Gmail or Google Workspace accounts/domains. If you did that please contact us.
3. You are logged in with a multiple emails, please log out of all accounts and log in only with the email account that your licensed is assigned. When you are logged in with multiple Google accounts, add-ons can work properly with only primary account. To workaround this issue/limitation, you can add different profiles on your browser and switch the profile to use add-ons with different Google accounts.