Why can’t I change my phone carrier

Your phone is locked to your previous carrier. Only the carrier can release the lock. Contact them and ask them to unlock your phone.

Is your iPhone XS Max carrier unlocked? Your iPhone may be locked to your previous carrier. Give your previous carrier a call and ask them whether your iPhone is locked to their network and if it is, then ask them about the requirements to get it unlocked. Once it is unlocked then you can use it on the T-Mobile network.

Axel F.