As your Mac is very old, I don't think you can use the native app anymore unless you follow this process How to fix synchronisation issues and start over and downgrade Joplin to 3.5 on all your devices (and never upgrade Joplin again until you stop using the Mac).
Alternatively you can use the Web app on your Mac instead, which will allow you to use the latest version. But as you're not using Joplin cloud, you will have to host the web app yourself to use it with Dropbox. See Joplin web app cannot sync with joplin server anymore? - #4 by mrjo118