Does the error notice prevent you from e-filing, or is it just a nuisance warning?  Apparently the payment of tax is billed separately by IRS, and the 965-A is useless paperwork that no one is going to look at anyway.  There's a June 2023 article about IRS problems with Section 965, here:

https://www.thetaxadviser.com/issues/2023/jun/sec-965-transition-tax-installment-payments-causing-pr...

It includes an IRS phone number which may or may not still be staffed.  

The Schedule 2 instructions don't even refer to Line 20, right?  So even for those who get hung up on IRS instructions being infallible, no one is going to jail by just leaving that line blank.  In my opinion, do your own research, etc.