I have tried different forms for which I have tax forms kite 1099r or 1099int but I get same message why?
So I was using NordVPN for web activity and an ad blocker Ghostery on Safari.
I had same message with tax forms import from Vanguard. One of the following solved the problem for me:
1. Clear all cache in Safari
2. Turn off ad blocking in Eero Router
3. Reboot Mac