For me the issue was the form being unavailable.
I found the answer in a different forum post:
https://ttlc.intuit.com/community/taxes/discussion/tax-year-2021-transmissions-are-not-available/00/...
After Googling around I found a page containing all of the forms' availability.
https://proconnect.intuit.com/proseries/forms-finder/
It would really be nice if the error message explained what the actual problem is, but TurboTax is becoming less user-friendly every year.
