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. 

Use Control Centre on Mac – Apple Support (AU)

Functional 12 Powerbook G4, Trying to Install OSX Panther

Apple locked my Mac Pro

If your Mac asks you to allow an accessory to connect - Apple Support

solved support.apple.com/mac/startup

Change a password or user picture on Mac - Apple Support

Get started with accessibility features on Mac - Apple Support

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

Mac Prohibitory Symbol Fix Circle with Line Through It Mac🚫