Why can’t I reinstall Mac OS high Sierra?

What keys did you hold down while you turned on or restarted your Mac? As noted by Kappy, Shift-Option-Command-R will boot the version of Recovery Mode that was shipped with your Mac. Use Option-Command-R to boot the latest version of Recovery Mode over the Internet. This will permit you to reformat your startup disk as AFPS and install the latest version of macOS.

Internet/Network Recovery of El Capitan or Later on a Clean Disk

1. Restart the computer. Immediately after the chime hold down the (Command-Option-Shift-R) keys until a globe appears.
2. The Utility Menu will appear in from 5-20 minutes. Be patient.
3. Select Disk Utility and click on the Continue button.
4. When Disk Utility loads select the drive (usually, the out-dented entry) from the side list.
5. Click on the Erase tab in Disk Utility's main window. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
8. Click on the Apply button, then click on the Done button when it activates.
9. Quit Disk Utility and return to the Utility Menu.
10. Select Reinstall OS X and click on the Continue button.

Note:

1. To install the version of OS X that was currently installed use Command-Option-R.

2. To install the original factory version when the computer was new use Command-Option-Shift-R.

I actually don’t know; I haven’t posted a question yet on this site. If you are unable to mark Kappy’s answer as Solved then at least mark his answer as Helpful. He certainly deserves it more than me on this thread.

If you want to reinstall and wipe your Mac, do this: How to reinstall macOS - Apple Support

Hello! You can tap the button: ‘This Solved My Question’ or some like that on his post. Oops!