[KLX678]

Key on mine is on the left side a few inches below the edge of the seat. Ignition key fits it.

If there is a problem inserting the key that should only happen if for some reason the ignition or the seat lock is not original equipment.

If the key fits in but is hard to turn or won't turn easily enough I'd agree to try to either push down on the seat while turning the key or give the seat a downward smack in the middle while turning the key. It is possible the seat wasn't put on properly, not engaging the seat hooks on the bottom in the proper spot on the frame causing the seat to push upward against the locking mechanism.

That's my best guesstimate. Seen the seats not placed properly on other bikes, like my 86 Nighthawk S and my 95 KLX650. Both required a bit of care when putting the seats on.