You can’t combine Azure Bulk Credit offers with other offers. If a user assignment isn't working, that probably means the user you want to assign is already using other licensing, prepaid, or credit offers such as Microsoft Enterprise Agreement, Cloud Solution Partner, Azure in Open, Azure Pass, Action Pack, etc. If the user isn't subscribed to any of these agreements or plans and you still can’t assign them, contact Microsoft Support.
For more information, see Azure and cloud benefits - MPN.