This was my thought as well. On my BX 2200 I believe the PTO must be off, the range selector must be in neutral, and maybe the directional pedal must be centered.
I would check these three items to be sure they
are in the proper position. Second I would try sitting in the tractor seat and holding the key in the start position, and moving these three items one at a time, and see if the engine started to turn over when any of them were moved.
Most important is that you give specific tractor model information. On my BX there’s a decal on the side of the frame that gives the information. If it’s the same on your tractor, take a picture of the decal and post it here.
edit: On my BX voltage is needed to keep the engine running. So it’s possible yours turning off when you removed the battery is not unexpected.