How to reset NVRAM
1. On your Mac, choose Apple menu > Shut Down.
2. Locate the following keys on the keyboard: Option, Command, P, and R. You will need to press and hold these keys simultaneously in step 4.
3. Turn on your Mac.
4. Immediately press and hold the Option-Command-P-R keys, before the black screen with the Apple logo appears.
5. Continue pressing the keys until your Mac restarts at least twice, about 60 seconds.
6. Release the keys.
When your Mac finishes starting up, you might want to open System Preferences and adjust any settings that were reset, such as sound volume, display resolution, startup disk selection, or time zone.