Files are "forward compatible" meaning the 2021 version can open a 2020 file. But they aren't backward compatible - this is because the features added in the new version were unknown when the old version was created so it can't deal with them.
There is a possible workaround - you can export the 2021 version for QuickBooks Windows. The resulting file can be opened in the 2020 version of the Mac file. It will lose Mac specific features like attachments and other Mac only prefs. But the underlying financial information will be intact.