Why won't the drill switch it's direction of rotation?

There are only a few things that could cause this issue, all of them having to do with the direction of rotation switch.

Check the direction of rotation switch for debris and anything that could be hindering its normal motion. If nothing is stuck, and the switch is free to move, but the direction of rotation doesn’t change, then it’s time to replace your switch. If you replace the switch and it still isn't working properly, there must be an issue with the wires somewhere. Try and see if any wires are stripped or disconnected. If they are then you may have to replace them or reconnect them if possible. This question and more can be answered on the RIDGID X4 18V Lithium-Ion Impact Driver Troubleshooting page.