after installing Windows on my MacBook Air using the Bootcamp app I cannot switch back to the MAC OS. When I try to reboot I get the message "could not locate the OS X Reboot Volume.

Hi CharlesT123,

I'm guessing you're using the option in Boot Camp to "Restart into OS X" or "Restart into macOS"?

If that's the case, please run Apple Software Update in Windows. There should be an update for Boot Camp that fixes this issue.

In the meanwhile, try using Startup Manager to switch between macOS and Windows:

1. Power on or restart your Intel-based Mac while holding down the Option (Alt) key.
2. Release the key when a series of startup volumes appear onscreen.
3. Use the arrow keys or the mouse to select which volume to start up from.
4. If you want to set the selected volume as the default one, hold down the Control key.
5. Press Enter/Return or click the arrow to boot that volume.
   1. If you held down the Control key, your Mac will always boot from this volume.
   2. If you didn't hold the Control key, your Mac will go back to the default startup volume on the next restart.

You may have better responses here: Windows on Mac - Apple Community