What is the nextcloud webDAV URL ?
I'm getting an error saying "WebDav directory not found "
https://joplinapp.org/help/#nextcloud-synchronisation
...Then input the WebDAV URL (to get it, click on Settings in the bottom left corner of the page, in Nextcloud), this is normally https://example.com/nextcloud/remote.php/webdav/Joplin (make sure to create the "Joplin" directory in Nextcloud), and set the username and password. If it does not work, please see this explanation for more details.