My second screen says "No bootable devices found"

I can only view on one screen because the other one is saying "No bootable devices found".

This is because your second screen was once a computer, but has now been configured to be just a screen. You should NOT try to turn-on the screen using the computer on/off button, but instead click the Source button and choose your laptop as the source. Note that the Source button should be highlighted by a red arrow stuck to the right-hand side of the screen.