Mac 2021 backup to Mac 2020 (saving the file for a client that uses older version)

You are unable to downgrade your file version. You may need to buy a 3rd party service to do so or ask your client to buy a new one time license for QBD 2021 Mac to upgrade the version. 

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.