Help Desk: PRAM/NVRAM Reset
How to reset NVRAM
Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P, and R. You can release the keys after about 20 seconds, during which your Mac might appear to restart.
*On Mac computers that play a startup sound, you can release the keys after the second startup sound.
**On Mac computers that have the Apple T2 Security Chip, you can release the keys after the Apple logo appears and disappears for the second time.