Unable to restore macOS High sierra

You must be getting up to Step 5 since what you stated indicates you have clicked on Erase tab in the Disk Utility toolbar. You must have an SSD if it automatically selects APFS. If you click on the Apply button it should work and format the SSD. If it now fails with the error about unmounting then your SSD is bad. Make an appointment at the Apple Genius Bar for service. If you need to find an Apple Store - Find a Store - Apple.

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

If possible backup your files before proceeding.

1. Restart the computer. Immediately after the chime hold down the (Command-Option-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.

If you followed the steps as written and still have a problem, then I'd guess the SSD needs to be replaced. Maybe you need to try more than once which may end up working. Exactly what model do you have? To find the model identifier open System Profiler in the Utilities' folder. It's displayed in the panel on the right. You may also find the complete model information by selecting About This Mac from the Apple menu.

Are you using a single SSD or something else? If something else then what? Were you able to contact Apple's installation server via Steps 1 and 2?