itās an overlook on their part, they removed the possibility to export to an older version, but forgot this type of messages.
someone else had the problem here
and the solution I gave them is that same Iāll give you
(extension to save as 2020)
before 2021, the files were not as compressed as they are now, thatās why in some very specific cases SU recommend going back to 2020.
the alternative would be to open your file, copy all, open a new file, paste all.
you will however loose your scenes.