There are a few reasons this might happen:
1. You are using a different email address than the one you purchased your license with.
Change your license from the Licenses page in the web app.
2. You used an email from another email provider than Google in the licenses panel.
The add-on licenses can be used only with Gmail or Google Workspace accounts/domains. If you did that please change it to your Google email address.
3. You are logged in with a multiple Google accounts.
Please log out of all google accounts and log in only with the email account that your license is assigned to. When you are logged in with multiple Google accounts, add-ons can’t work properly properly.
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 or use a incognito/private window.