If you've successfully
Prepared a template
Have enough credits in your account
but still end up getting an error:
"Signature has not been set due to an error (please check in Google Workspace that the service is activated for the user and the address is valid)"
that looks in Dashboard like this:
The most probable problem is that the user, whose signature you tried to set, is in an OU in Google Workspace that doesn't have SignatureSatori enabled.
Another frequent option is that the new user has not logged in yet and has not activated Gmail.
If you're not sure how to enable a Marketplace app to a specific OU, you can find more information in this article.