Lexa4 It sounds like you might have bought a locked phone. USA-based carriers (T-Mobile included) are known to do this.
But just to be sure, connect your phone to the internet if you already haven't. Some Pixels need an internet connection to check whether they were sold as locked units ot not. Maybe there's a chance your phone isn't OEM locked after all.
But if that doesn't enable OEM unlocking, then you're out of luck and your phone is a locked one. Nothing that can be done about it at that point other than selling it and buying an unlocked Pixel elsewhere (not through any carriers or carrier shops).