An update was announced, which I ran. Once it was installed, it was announced that my version of Mac OS is too old. Is there a repository from which I can download and install the older version?
You may also need to ensure you don't use any Joplin 3.x clients at all (mobile etc.) if you sync because changes may be made to the sync target that your older client won't be compatible with. For mobile apps this would usually mean turning off auto updates.