What tool do I need for this bolt that holds the crank arm on this stationary bike? I'm amateur and got this stationary bike. The crank arm, I think that's what it's called, fell off. I can put the bolt back (the one inside the red circle) and turn manually but can't tighten it. There was a cap on top of it. What tool do it need here?
It's completely round and not hex shape
That nut was put in the wrong way around.
This is what the nut looks like:

The hex end is what you should tighten with a approriate socket.
This is what it should look like when installed correctly:

If it ever get's super tight as it is now, to get it off, the easiest way is probably to cut it with a tremmel and chisel it out.
Good luck!