Accidentally erased operating system - trying to "reset to factory settings", now unable to reinstall. Help please!

I tried to follow some steps online to erase the data from my macbook pro 2017 to prepare it for sale, but via disk utility, appear to now have erased the operating system.

I have tried the Command + R to get to the reboot system, but:

• Cannot reinstall from time machine backup as no backup sources appear
• Reinstall macOS Sierra onto the HD experiences an error at the final stage 'an error occurred while preparing the installation. Try running this application again.'
• Using Disk Utility I can click on Macintosh HD, but the partition option is greyed out and the first aid process fails 'If possible back up the data to this volume'

I'm very stuck if anyone could help please!