The User Permission page in the DocGen Admin tab searches for Users who do not have a DocGen Permission set (Admin/User)
Please check to see if the User which is being added does not contain a manually added Permission set.
1. Navigate to the User and If they contain a DocGen Permission set, remover it.
2. Navigate back to the DocGen Admin Tab and the User should now be available.
3. Once added the User will obtain a Permission Set and a Seat on the License.