Trade-in in store question

In the past I would try to complete a trade in in person after a phone was shipped to me and it worked fine. Then they would always say there’s an account error.

The current reason is now that because I ordered it through Costco in the warehouse, a Corporate T-Mobile location can't process my trade in. They’re trying to say it’s because it’s a third party but aren't Costco and Sam’s Club just corporate T-Mobile locations?

Is this true that I can’t trade in a phone in person for an order from Costco or is something wrong with their system/my account?