It will lock with the head raised. There doesn't appear to be anything obstructing it from sitting flat, but it doesn't want to lock.
Lift the head all the way up. On the top of the stand, just next to the mixer itself you will see a little square hole. Check if that hole is obstructed.
If it looks clear, then check that the set screw on the right side of the stand is not missing (as suggested here)