Timing Chain Installation Question

OK ! Been there and had the same prob b4. First thing make sure there isn't any burrs on the gear keyway or the key itself from previous attempts. Next start to install as usual again. when you just get it started, and the key starts to move - take a long pin punch and tap the key back down at the same time tap the gear back on the snout of the cam (at this point a friend might be good to have ).
Also sounds like the key might be a tad too small- you should have to tap (lightly) the key into the keyway -if youy can push it in with your thumb it is too small.

Also trial fit just the gear on the key and cam. slidre it off and on a few time before doing it with the chain