Thanks for asking the Apple Support Communities for help with your Mac. I am happy to check this out with you.
I recommend using macOS Recovery over the Internet. This article will help: About macOS Recovery - Apple Support
1. Start with the Mac shutdown.
2. Turn it on and hold down Option-Command-R or Shift-Option-Command-R at startup.
3. After it loads up to the Utility screen, choose the second option to Reinstall macOS.
4. Point the installation to the internal drive. It should be names Macintosh HD unless it was ever changed.
Thanks and have a great evening.