If you've successfully
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.
Another frequent option is that the new user has not logged in yet and doesn't have 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.