I transferred my service from AT&T to Verizon

Probably you have a locked iPhone

Carrier Lock:- Request your iPhone’s original Carrier to unlock your iPhone and if they agree and do it, then you may know the further process here --> How to unlock your iPhone for use with a different carrier ...

Device Unlocking Policies | Verizon Privacy Policy

Unlock a Phone or Device - AT&T Device Unlock Portal

Unlocking your Sprint device - T-Mobile

You have two tasks.

1) Call AT&T and ask them to remove the SIM lock on the phone. Only AT&T can remove the lock. Apple cannot do this. AT&T may be less than thrilled to do this and depending on the original sales agreement, they may be under no obligation to unlock the phone.

2) When AT&T has unlocked the SIM card, call Verizon to get them to help set up the phone on their service.