The native nextcloud support with Gnome is not supporting notes yet. In order to sync your nextcloud notes with your gnome desktop you can either use gnome notes, which in my opinion is neither stable nor comfortable, or, my preferred method, sync with qownnotes.
You also need the owncloud syncing tool for files. For debian you can find it here:
https://software.opensuse.org/download/package?project=isv:ownCloud:desktop&package=owncloud-client
