Mr. Yamaki, maybe you can try a hand technique to release the clutch... Tie the clutch pedal down like it's constantly pressed, then put it in 2nd or 3rd gear to begin (not the shortest one, but in the middle), then grab the rear wheel on top and jerk it forward and backward, since you have always enough play between gear teeth. You don't need to be too rough though. If it still doesn't release after some time, then you might have some mouse nest in the bell housing, if there are some openings big enough. If mice piss oo the clutch plate it might get almost permanently stuck.. Keep on, good work, but be careful with tractors. 👍