How do I fix a corrupted Mac OS?

From the macOS Utilities window, select Disk Utility, and click Continue. Select the APFS or HFS+ disk or volume that you wish to repair from the sidebar of Disk Utility. Click the First Aid tab, then click Run. Let Disk Utility repair your corrupt SSD.

Why is my copy of macOS Mojave damaged?

If you are getting an error similar to “This copy of the Install macOS Mojave. … The cause of this error is an expired certificate, and because the certificate is expired, the “Install macOS” app for Mojave, Sierra, and High Sierra will not run.