I don't think the shift lever locks with the ignition as in newer cars. On my '72 I can move the lever even with the key out.
Are you sure you are pulling it toward yourself all the way?
On mine the bushings have some play, so when one pulls the lever it comes
a few inches and 'stops'. But it won't shift. Then it has to be pulled back more against a fairly heavy spring to actually unlock it and then it can be shifted.

Csaba
72 280SEL 4.5