Seat latches on these are fussy and need adjustments
to make them work at all.
Try to loosen the seat nut or bolts near the front
and then turn the key counterclockwise firmly and
wiggle the seat around up and down or whatever
until it releases. The cables on the latch
may be broken or need adjustment to pull
the latch open. When you get the latch and/or the
cables set then tighten the seat bolts in that position.
You might have to push down on the seat and then
turn and hold the key firmly counterclockwise
and then lift the seat again while holding the key over.
If you get it open then try adjusting the cables so they
pull the latch more to the open position.
It can be difficult to get to the latch and manually
push or pull on the latch release. A coat hanger
might help or whatever works. There is no other way
to get the latch open so it might take some patience
Let me know how it goes.