I understand your Mac was turned off during the install process and now you're getting the prohibitory screen. I would suggest reinstalling the macOS, here's a few helpful articles.
About the screens you see when your Mac starts up
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 OS X. Contact your IT department for more help.
If this is your personal Mac, try reinstalling OS X by using OS X Recovery.
Here's another article: How to reinstallmacOS
You can install macOS over the same version or earlier version, without removing your data. You don’t need to remove or disable the existing system first.
Let me know if this helps.
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