You probably have a couple of things going on. First the cable has stretched and the lever on the floor won't open the trunk. Here's how I fixed mine when that happened to me.
Trunk doesn't pop up using latch beside the driver seat
The other thing is that once you get the trunk cover off, and the latch unbolted from the trunk, you will see that there is a rod that connects the lock mechanism to the latch mechanism. The rod is held in place by a cheapie plastic clip. Mine was lime green, YMMV on the color. Anyway if this plastic latch cracked, then the rod will come out of the latch easily and now the key in the lock won't unlock the trunk.
Try pushing the lever on the floor to the right towards the center of the car while lifting up on the lever. Fiddling with it should open the trunk. Then follow the instructions on the link and you should be all set. BTW, the first time I did it, I only used one nut, and it wasn't enough, so I had to go back and add another one later. Save yourself some trouble, and use two the first time.
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