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The System Files of your Mac must be corrupted. We can try some basic troubleshooting steps and see if it works or not. First try Disk Utility.
If that didn't work, then reset the PRAM and NVRAM and also try to restart in Safe Mode.
Reset PRAM and NVRAM
Restart in Safe mode Make sure that the computer is shut down. Press the power button. Immediately after you hear the startup tone, press and hold the Shift key. Release the Shift key when you see the gray Apple and progress indicator. Let the Mac boot up...
Let me know if that didn't work, so we continue troubleshooting. Don't Rate my answer until you are satisfied.