Installing developer gmail add-on on G Suite account

Question

I'm trying to install a developer Gmail add-on on my G Suite account (the email address does not end in "gmail.com"). The add-on was developed and tested using a gmail.com account.
However, when I insert the development ID into the "Developer add-ons" field and click install, instead of the name of the add-on I'm getting this error message:

This developer add-on is restricted by policies within your organization.

What policies could restrict a gmail add-on and how do I change them (...or the add-on to adhere to the policies)?

Answer 1

Go to Admin > Marketplace Apps > Google Script and set ALLOW for you or whichever users you want to install.

Answer 2

If it is an unpublished addon, it must have been created by a developer from the same domain (i.e. has an @your-domain.com GSuite email address). If it does not, duplicate the appscript file so you own it, and try adding it then.

Relevant part of the docs: Users in a G Suite organization can only install and run unpublished add-ons authored by users in the same organization.