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. 

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

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

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

Use Siri on Mac - Apple Support

Erased HD - Apple Community

Help me regarding issue 2106F when I trie… - Apple Community

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

MacBook Pro suddenly stopped working, the technician says the problem is not in the hard disk, happened to someone? : r/mac

2010 Macbook Air failing startup - Apple Community

Mac Smart People I Know

How to fix support apple.com/mac/startup

Erase your Mac and reset it to factory settings - Apple Support

Set up content and privacy restrictions in Screen Time on Mac - Apple Support

macOS Recovery - Information Technology Support - Answers

What does this problem means? : r/hackintosh