The pics of shaft is not enough info. Whats behind that corrugated sleeve cover? there could be a band or something. It looks like it might be flexible enough you can roll it back to get better access or remove that plastic sheild so you can have better grip and underestanding how it works so it can be reinstalled. what about pics of whole shaft? there is something holding it, gotta be the silver collar to let go of ball inside and then you can slide it off.
If you haven't already called the dealer to come back and show you the right way to remove/install that shaft, go make that call!!! All dealers should be showing you the inside and out of equipments or answer questions you have.
If for any reason you still making efforts to work on it yourself, try shifting the pto selector to MMM only so the shaft will spin freely and turn it to take a good look all around. its possible due to angle of tiller on ground, its binding the lock collar so it won't slide one way to unlock the coupling.
try holding the shaft towards tiller and push in and pull back collar then pull whole thing out. pls resist the urge to put screwdriver on the silver collar, everything is designed to be worked with human fingers pressure.
good luck, hope you get it figured out. ( I just wish there wasn't so many variations of pot coupling locks, it'd be easier to troubleshoot for you)
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