Sometimes the gear puller threads are not manufactured properly. The solution is to purchase another gear puller; allowing the tool to thread all the way into the 10 tooth output shaft sprocket.
Because it uses a fine thread, it's easy to cross-thread the tool into the sprocket; making it much more difficult to thread a new puller tool into the partially damaged sprocket threads, if that has been the case.
It's the one kind of f*#kup that you really want to avoid, should you have previously cross-threaded the tool into the sprocket.
No real way of easily getting around the problem without using the gear puller tool and without (potentially, and especially if you are a newbie) creating damage to the output shaft and/or the output shaft bearings.