If your Mac starts up to a circle with a line through it - Apple Support

A circle with a line through it means that your startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. 

How to fix globe icon with exclamation mark alert on Mac

New to MacBook - questions about how to d… - Apple Community

Mac Smart People I Know

Use templates in Pages on Mac - Apple Support

Use Siri on Mac – Apple Support (UK)

Navigate your Mac using Full Keyboard Access - Apple Support

If you can't reset your Mac login password - Apple Support

Circle Slash No Entry Error - After Update MacBook Pro / Air

Crossed out circle at boot up. Recovery n… - Apple Community

Cannot boot iMac (late2015) - Apple Community

If your Mac is lost or stolen - Apple Support

Mac boots to a circle with a line through it after resetting

Terminal User Guide for Mac - Apple Support

Search for emails in Mail on Mac - Apple Support