If you've succesfully
but still end up getting an error
"Signature has not been set due to an error (please check in G Suite 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 G Suite that doesn't have SignatureSatori enabled.
If you're not sure how to enable a Marketplace app to a specific OU, you can find more information in this article.