Take a look at FAQ — Syncthing documentation . The user running Syncthing on your device has the rights to write a folder marker in the Syncthing user’s home directory, but not in the other directories you want to sync.
The fix is to give the Syncthing user the appropriate write access to create the folder marker. If you pick another file to be the folder marker (also at the link above, just under “Usage”) you will probably run into problems where the Syncthing user cannot write new or changed files to those folders.