A prohibitory symbol, which looks like a circle with a line or slash through it, means that your startup disk contains a Mac operating system, but it's not a version or build of macOS that your Mac can use.
- Press and hold the power button on your Mac for up to 10 seconds until your Mac turns off.
- Turn on your Mac and immediately press and hold both Command (⌘) and R to start up from macOS Recovery.
- While in macOS Recovery, use Disk Utility to repair your startup disk.
- If Disk Utility didn't find any errors or has repaired all of the errors, reinstall macOS.
- If you still need help, please contact Apple Support.