I am using Ubuntu Mate 19.04 and Gnote. Its help indicates "In order to use synchronization, you have to configure it first in the Gnote configuration dialog.".
There is no configuration option in the few available options from the "hamburger" menu (new note, new windows, grayed-out Sync..).
Where is this configuration dialog ?
I am using Gnote in Linux Mint (LMDE), PopOS 20.10, Ubuntu 20.10 and 18.04LTS. In all versions you can find the synchronisation configuration dialogue under the "hamburger" menu - Preferences - Synchronization tab. You have to be in the main windows of Gnote, i.e. the one listing your notes, not editing a note.