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. 

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

SOLVED: Circle with a line in it ,in the middle of screen

Organize files in folders on Mac - Apple Support

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

Mac Basic Startup Troubleshooting Steps - Apple Community

How to Fix Mac Boot to a Circle with a Line Through it?

Sign in or out of your Apple ID on Mac - Apple Support

Mail app not working on Mac? Here's how to reset it

MacBook Pro m1 random exclamation mark with circle : r/macbook

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

Terminal User Guide for Mac - Apple Support