As I’ve mentioned previously, spctl is the command line tool to manage signing for Gatekeeper. This file edits the information stored in /var/db/SystemPolicy. Regrettably, this information can become corrupted (like all information in a database. And when it does, Apple has done us a favor by making a hidden default database with this information in the same directory, called .SystemPolicy-default.
To restore the default database, we just copy it over the top of the old one. Here, we’ll rename the old one first:
mv /var/db/SystemPolicy /var/db/SystemPolicyOLD
And then we’ll copy the defaults to make it the production database:
cp /var/db/.SystemPolicy-default /var/db/SystemPolicy
Then reboot and you should be good to go.