macbook air crossed out circle on a white background

Hi there,

See this instructions on how to fix your MacBook

A prohibitory symbol, which looks like a circle with a line or slash through it, means that your startup disk contains a Mac operating system, but it's not a version or build of macOS that your Mac can use. 

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Use Disk Utility to repair your startup disk. 
3. If Disk Utility found no errors or repaired all errors, reinstall macOS.
4. If you still need help, please contact Apple Support.

I have this same issue on my 2012 MacPro5,1 w/ EVGA GTX680 Mac Edition. It started during a re-installation of High Sierra, which is absolutely compatible and therefore, should not have this issue. Trust me, I have even had an EVGA RTX2060 installed and never once saw this issue. So, in my opinion, this points to another issue, which is not apparent at this point.

If someone has this issue with no good explanation, please post any valid information, regardless of post date. We are all customers and some still use older yet valid hardware.

Update: I did find this youtube video: https://www.youtube.com/watch?v=EQmWR96FiLg relating to this issue.

Thanks for posting this @AlinaKhanamirian,

Cheers