Messed up my /private folder!
If you have a backup and want to learn how to finesse this, try booting in macOS Recovery. Then:

From the Utilities menu, select Terminal.

Run:
  chmod u=rwx,go=rx &quot;/Volumes/Macintosh HD/private&quot;


Reboot.


This will set the permissions to read-write-execute for root and read-execute for anyone else.

Alternatively, try this or see the answer linked by @grgarside (this).
If you don't have a backup or don't want to make things worse — from macOS Recovery, a quick reinstall of the OS will re-write everything that's critical for the system to run without erasing any programs or user data. Don't mess with anything but a clean reinstall unless you are sure exactly how you messed things up and are confident you won't do more damage trying to fix it by hand.