I had the same problem. Tier 2 support confirmed that the fix was to enter the value from box 2a into box 16. This cleared the error condition and allowed me to e-file.
Also, when asked about the mismatch between my filled in box 16 and the empty box on the 1099-R sent from my financial institution, the reply was that this was ok and that it represented, "Standard procedure."
I'm not sure how this represents standard procedure because 2025 is the first time this has arisen for me.
Re: Box 14a? It seems that there can be multiple entries in Box 14 so the first entry is referred to as 14a. Finally, I can tell you that filling in box 16 made no difference to my final Federal or State numbers.
I hope this helps.
