Most likely the cable has stretched or come loose and it will not unlock no matter what you do.
Don't know where exactly the 2016 seat latch is located. I have some pictures of a 2019 seat latch with an emergency release cable on them. You should have put an emergency release on it when it first began to be a problem. It is a known problem and only gets worse the longer it remains. If you have a maintenance shop manual for the 2016, or know someone who does, there should be a picture in the shop manual.
These pictures are for 2019 cable from key seat release. the basic seat latch should be the same, but exactly where it is located will be different depending on make and model.

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That last thumbnail is a screen shot, so the links will not be active. However, you can copy them and type them into your browser and it will still take you to the discussion about releasing a stuck seat latch. It can be done with a flashlight and coat hanger, or maybe a hook made from bent brazing rods. It is difficult and requires some fishing around under the seat. Once you get it open, it is best to disconnect the cable from the key, and instead, put a cable loop in it that you can pull from the side of the seat.
Also read the last thread that had the same problem, started by
@Poasttown -
My key will not unlatch the rear seat on my 2019 F3L. I have removed the seat many times, but now, when I push the key in and turn it right, it does not unlatch. I placed my hand on the key switch under the tupperware and can feel the cable moving. I think I know approximately where the latch is located under the seat, but I can’t see it. I have completed all the suggestions I could find by searching with no success. There were a few similar threads in the past, but for some reason, I didn’t see any mention of what they did to actually get the seat unlatched. Any suggestions would be...
Active link this time.
If this is not enough to get the seat to release, post back with specific information on the type of release you are dealing with. We'll need to know if it is one of the key to cable inside cover latch releases, or if it is electrical operation. What action do you take normally to unlatch it?
If all else fails, there is a way to cut the cable inside the cover in some area that can be exposed by removing the tupperware and pulling on the cable with vice grips. It will require destroying the current seat release cable operating system, but it is most likely beyond repair by adjustment and lubrication anyway. I have removed all the 'cable from key' operating latches from my 2019, as have a lot of other owners, and many of those who have not removed them completely have installed emergency release cables that are stuffed up under the edge of the seat.