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

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. 

Apple locked my Mac Pro

Stuck on circle line logo after disabled unsupported gpu. : r

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

Mac shutting down for no reason, and unab… - Apple Community

Late 2010 Air Not Able To Connect To Reco… - Apple Community

Mac Basic Startup Troubleshooting Steps - Apple Community

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

Startup issue. PLEASE HELP - Apple Community

Mac Mini won't restart after Mail crashed… - Apple Community

Mac OS install on AMD Ryzen (VMWare) Opencore

2010 Macbook Air failing startup - Apple Community