Can't upgrade to the Samsung Galaxy S21 Ultra

[niltag]

I bought a new Samsung Galaxy S21 Ultra (Unlocked) direct from Samsung.
I have a new BYOP sim card.
I tried to do the upgrade / activation but it didn't work.
It said something about the phone not being compatible?
Is this just a issue of the phone being so new that the IMEI has not been updated into whatever server is checked for those things? If so, how long does that take to update?

I really want to stay with Straight talk, but will switch if that is what it takes to use my new phone.

Thank you
Wendell