If anyone is still stuck in the loop between iD Mobile and Apple regarding the "UID Missing or incorrect" error for the Apple Fitness+ trial, I managed to get it sorted today.

The likely cause of the error: iD Mobile is advertising this as a 2-month trial, but the Apple advisor I spoke to confirmed their system only has it logged as a 1-month promotion. It seems highly likely the link is breaking because iD Mobile's system is trying to force a 2-month redemption through an Apple API that is only configured to accept 1 month.

How to fix it today (without waiting for iD Mobile):

1. Don't bother calling. Go directly to Apple Support Live Chat. (If you can't find the chat option under 'Billing/Subscriptions', select a hardware issue like 'iPhone > Battery' to force the live chat option to appear).
2. Tell the Apple Advisor: "I have a broken carrier promo link from iD Mobile giving a 'UID Missing' error. Can you issue a manual code instead?"
3. The advisor understood immediately, abandoned the broken link entirely, and generated a manual code that I typed straight into the App Store.

Note: You will only get 1 month, not the 2 months iD Mobile advertised, but it bypasses the broken link entirely and gets your account active immediately. Hope this helps anyone else tearing their hair out over this!