You can’t change the startup disk to the selected disk?
The first thing I'd try is resetting the NVRAM:


Shut down your Mac.
Locate the following keys on the keyboard: Command (⌘), Option, P,
and R.  Turn on your Mac.
Press and hold the ⌘+Option+P+R keys immediately after you
hear the startup sound.
Hold these keys until the computer restarts and you hear the startup
sound for a second time.
Release the keys.


...and see if that solves your problem. If it doesn't, I'd start worrying about a hardware fault.