Support.apple.com/startup How do I fix

If your Mac doesn't start up all the way - Apple Support

If your Mac starts up with a screen with a prohibited symbol, your harddisk is containing a macOS operating system, but it is not a version of macOS your Mac can use.

Please follow these steps to see if you can fix the issue

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Use Disk Utility to repair your startup disk. 
3. If Disk Utility found no errors or repaired all errors, reinstall macOS.
4. If you still need help, please contact Apple Support.