I made a gym with an elevator to get to the 3rd floor. My sim won't go in it.

I deleted the shaft and turned off moveobjects on. When I tried to put the shaft back in it wouldn't be placed, so I put flooring in the 2x2 sized room on the second floor. My sim is still unable to enter the elevator.

Yeah, I went with common sense because elevators can't go up when there's flooring above, so I left all of the 2x2 rooms floor-less (except for ground level of course)

But I will see to all of this and get back to you

If you still can't make it work, remove all the furniture close by, demolish walls around the shaft. It's usually what I do to trouble shoot. Then you might find what's causing it, and can put things back.

If you're using cc close by, they may have effect where they shouldn't.

I must say my instinct tells me you have walls like a hallway by the elevator entrance, I would have liked it to have more room by the entrance.

I've found when building with an elevator, best thing to do is make your building with just the floors, put in the elevator shafts, wall it in then build the rest of the floors. Lastly decorate and you should be alright.