This was what I did :
- Make sure you are connected to wifi or network.
- I restarted in Internet Recovery Mode by holding down Command+Option+R. On windows keyboard this is Windows Key+Alt+R
- This will take sometime so wait for it
- Once done it will take you back to normal recovery mode.
- Run First aid on the disk you want to install to.
- Click on your Main Disk where you want to install High Sierra then convert it to APFS (Edit -> Convert to APFS)
- Once converted, click on Reinstall MacOS. It will prompt you to install High Sierra. Then proceed to install MacOS High Sierra.
All my files and applications are still there. And it worked. Well, do make sure you have a backup before attempting to install High Sierra.