I'm having the same problem.
We had to amend our 2020 return, so the AMT credit carryforward TurboTax imported from the last year's return is now incorrect. In 2020 we did not owe the AMT, so then it worked to use AmyC's suggestion to go into the "credit for AMT paid in a prior year", but in 2021 we do owe AMT and attempting to go into that section now just results in it saying "We checked and this credit won't affect your taxes this year. For next year, you have a $<incorrect> carryover" and does NOT let you enter the values from the prior year.
I don't understand why it doesn't just let you update the prior year Form 8801 line 26 value directly when editing Form 8801? You can edit the other fields from the prior year (6251 line 1, etc.) in that form.
