If your Mac starts up to a circle with a line through it
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.
- While in Recovery, use Disk Utility to repair your Mac startup disk. 
- If Disk Utility finds no errors or repairs all errors, reinstall macOS from Recovery. 
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