I thought I was discarding my 2017 MacBook Air. So I reset it and deleted files. Now, I need it again! All I see is a black screen with a flashing question mark. Guess I removed the Sierra operating system and all files. Can I reinstall Op System?

What happens when you press ⌘+R immediately on a reboot? Are you presented with a menu offering to Reinstall the operating system, or Restore from a Time Machine backup? If you get that far, and you have a Time Machine backup of the operating system that you just wiped, you can choose that entry and your drive will be repaved just the way it was before you whacked it.

If the first paragraph is a bust, then use an appropriate startup command shortcut to get the operating system from Apple's servers. Be connected to a power supply and have a network connection.