MacBook Pro 9,2 shows circle with diagonal having accepted High Sierra upgrade

From the Article that panchososa694 linked you earlier this is the section you will want to follow:

Prohibitory symbol

When you see a circle with a slash symbol instead of the Apple logo, it means your Mac couldn't find a valid System Folder to start up from.

If you're using your Mac at a school or business, it might be trying to start from the wrong version of macOS. Contact your IT department for more help.

If this is your personal Mac, try reinstalling macOS using macOS recovery. (specifically this as your system will need a reinstall of High Sierra)

Hey Boncle,

If you are ever getting the prohibitory symbol at any point in most cases a OS reinstall is needed.

Boot the mac into macOS Recovery by powering on the computer and holding CMD + R to boot into recovery mode.

Once there you can select Reinstall macOS High Sierra from the menu, it should be the 2nd option.

Good Luck!

Also make sure that you have your Apple ID available as when you go to download the installer again you will need you login with your Apple ID information

https://support.apple.com/en-us/HT204156

Hi just tried your instructions for the same problem on our Macpro but it says the recovery server could not be contacted

I believe that signals a network problem. Make sure you're connected to the internet and it wouldn't hurt to reset your router.