This is the third case of this I've seen in the last week. It's more common than I thought.The clutch might be out of adjustment, but more likely the plates are sticking from long storage.
If you remove the clutch cover, (RHS), then pull the clutch in, you should see the outer plate move outwards slightly. If you do, clutch adjustment isn't the problem. The plates are sticking.
In this case, you'll need to remove the top clutch plate, then re-test. If it's still sticking, sing out.
To remove the top clutch plate, first pin the clutch in. Next, remove the Philips screw then undo and remove the clover nut, counting the turns if you like.
The top plate should come off. If not,
make it come off, as gently as possible.
Check if the clutch has disengaged and report back.
(Saves me typing more if it's not necessary.)
... Steve