Yup, same here, login for the arm patch claims that I didn't register for the beta on this account, but I did and also have a preordered copy of the base game sat in my library. The shirt just gives "an error" and there's not even an option for the weapon skin (which I assume is coming later). Played all the content over the weekend no problems and it shows as my most recently played game on the tracker so there's at least some kind of hookup going on, just not the right bits yet.
I put in a support ticket for this issue (t-shirt and arm patch still locked) and they basically told me they can’t do anything because beta is over.
Here is what they responded to my ticket with:
“Thank you for contacting Ubisoft Support.
Thank you for your interest in The Division 2 Private Beta, however the beta has now concluded and we are unable to provide further access to the Private Beta or troubleshoot any issues related to the Beta. We’re sorry we were unable to respond to your inquiry before the Beta ended. For any further issues or feedback you would like to report, please do so on the Private Beta forums here”
I responded back to them that I wasn’t looking for access to the beta because I know it ended. I need access to my rewards for playing the beta. No further response yet.