What operating system are you on? I don’t exactly know where it is on Windows or OSX, but on Linux, it should be in $XDG_CONFIG_HOME/joplin or $HOME/.config/joplin (depending on OS; many are moving towards $XDG_CONFIG_HOME, so I’d try that first).
Sorry, I really should have read the page you linked.
Nextcloud synchronisation🔗
You will need to set the sync.target config variable and all the sync.5.path, sync.5.username and sync.5.password config variables to, respectively the Nextcloud WebDAV URL, your username and your password. This can be done from the command line mode using:
:config sync.target 5
:config sync.5.path https://example.com/nextcloud/remote.php/webdav/Joplin
:config sync.5.username YOUR_USERNAME
:config sync.5.password YOUR_PASSWORD
If synchronisation does not work, please consult the logs in the app profile directory (~/.config/joplin)- it is often due to a misconfigured URL or password. The log should indicate what the exact issue is.
The config it's talking about here isn't the file itself. It's referring to setting these options directly in the terminal client.
Open joplin
Type : to open the command prompt.
Type each of the commands above and press enter to set and store them.
Are you only running terminal tools on your raspberry pi or have a GUI too? If the latter, you should be able to install sqlitebrowser and open database.sqlite from the config directory you linked above. I’m not exactly sure where in the file that information will be, but most, if not all joplin configurations and other information is saved directly to the database, fwict.