The following may not cover your exact case but should allow you to sort things out.
When you sync between computers you first set up one primary computer under FILE (MONEYDANCE on Mac) – SYNCING. You then set up syncing any number of secondary computers using FILE (MONEYDANCE on Mac) – NEW – OPEN SYNCED FILE.
You can change the syncing method on a primary data set or a Moneydance backup restored from a primary data set.
With a secondary data set the syncing method is fixed, greyed out and cannot be change. You get the same result after restoring a secondary computer Moneydance backup. Make sure the auto backups for the primary and secondary computers are not set to use the same location if you are say using Dropbox.
If you have lost your primary computer data set first try to find a backup created from your primary computer and restore from that.
If you only have a secondary data set try the following:
To convert a secondary data set to a primary data set you need to run the attached script md_convert_secondary_to_primary_data_set.py in Moneydance 2019/2020/2021.
With your data set open click WINDOW - SHOW MONEYBOT CONSOLE use OPEN SCRIPT to select the script and click RUN. Wait for the script to complete its run, it can take a minute. Close and reopen Moneydance. You should now be able to change the syncing method.