I have the same issue. I tried at opening, and just now. As little experiment, I tried a random number for the PIN and it told me that was not the correct PIN. So that confirms I'm using the correct PIN, but won't let me proceed because my account is supposedly inactive. Meanwhile, attempting to look at my profile on t-mobile's web site just spins, so that seems to confirm the issue is on their end. I'll blame them when I have to wait for my iPhones 14 to arrive after the initial arrival date. #firstworldproblems.
I looked at ordering directly from T-Mobile, but I didn't like the way they did the trade-in discount nor did they offer straight AppleCare+ or the accessories that I wanted.
There are worse problems in the world to have, but it's really disappointing that these tech companies can't have their tech ready for an event like this.