What tool do I need for this bolt that holds the crank arm on this stationary bike?
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!