First I would simple restart Windows and Lacerte. If still happens and happens for more than one client then go whole hog with https://proconnect.intuit.com/support/en-us/help-article/federal-taxes/error-getting-calculation-dat...

??? is this 2021 and did it happen after this latest update that I have not yet installed???

It’s 2022 and started after the early December update. 

From past experience, I do not do proforma until as late as possible. It is always full of bugs in the beginning. That may be the source of your problems. 

Try the new Lacerte Tool Hub https://proconnect.intuit.com/community/help-articles/help/fix-common-problems-and-errors-with-the-l...

Same issue here, however only within S-Corp.  KB article suggests repairing the database.  Not sure...

https://proconnect.intuit.com/support/en-us/help-article/federal-taxes/error-getting-calculation-dat...

If this is 2022, I would consider deleting all clients and doing the proforma from 2021 again. Backup any 2022 that you have "real" work in, but toss the others. 

I have not yet done proforma, just because of fears like this. 

An update came through today (or late yesterday, I left early because Lacerte was not working) and appears to have fixed the issue.