You already have a subscription through work

PrinterGus240 That error message indicates that the tenant you're applying the promo code to already has Universal Print enabled.

You should already be able to use a global administrator account to assign Universal Print licenses to users in the tenant, and a user with the Print Administrator or Global Administrator role should be able to register and manage printers.

If you have trouble assigning licenses or registering/using printers, please reach out to Microsoft Support and they'll make sure you quickly get the help you need!