You need to upgrade to v20.9.89, which was the last version that had the necessary conversion script. See Business database not valid, desktop version on Windows 10 - #2 by lubos.
Because you have tried to open your file(s) with a newer version, you may be forced to add an older backup first. The backup will be converted. So add the backup first, then install v20.9.89 and open the backup, then install the latest version. You may or may not be able to open your current live database, depending on what else you might have done to try to get things running.