UPDATE

I have Quickbooks Pro Desktop 2012 and was trying to upgrade to Quickbooks Pro Desktop 2021 (nonsubscription).  The problem was that QB2021 would not generate the Tax Forms to Excel function but instead gave an error message.

I called Intuit support but all I got was scripted "uninstall this" and "reinstall that" recommendations.

I started testing.  I noted that the specific file that QB2021 was trying to load appeared to be QB_Taxform_Link.XLT (file size 854KB and date 10/11/2021) which was in the QB's template directory.   When it executed, it would start and allow me to select which tax form I wanted to generate but then it would generate the same error message (see attachment) no matter which report I selected.  

I then looked back to see whether that template was also in the QB2012 template directory.  A file of the same name was there but it had a different size and date.  So, I renamed the template file in the QB2021 directory and copied the file of the same name from the QB2012 template directory into the QB2021 directory.  I reloaded QB2021 and ran the Tax Forms to Excel function from within QB2021 using the QB2012 XLT file and it worked.  I confirmed it was hitting the QB2021 datafile(s) by editing the last payroll check and adding $10,000 to the amount and regenerating the report.

I called Intuit support with this new information, but all I got was the script readers again.

Finally, I logged out of QB2021, I shut down all the QB services that I could identify, I closed all Microsoft office files and I loaded QB2012 and reran the Tax Forms to Excel function with the original QB2012 XLT file.  It processed as it has the last decade.  I then edited the last paycheck in the QB2012 database and ran it again and confirmed that the function was  hitting the QB2012 datafile.  I edit the paycheck back to its correct disbursement.

When I went back into QB2021 and confirmed that the QB2021 XLT file was in place, I tried the Tax Forms to Excel function again but this time it worked.  I have no idea what changed or why it worked but it did.

I would like some insight from Intuit as to this situation because I want to confirm operations before migrating fully from QB2012 to QB2021.