Usually the reason this happens is the user was not added to the group. You may have added them to the account by mistake. See Add Users for instructions to add the user to a group.
If you have added them to the group and they still cannot create codes, make sure that you gave them User or Administrator access. If you add them as an Edit Only or Read Only user, they will not be able to create codes.
If they have not registered on the Ad-ID system, they will receive an email invite with a link to the Ad-ID registration page.
If you continue to have issues, please contact customer service.