I did an erase on my mac book air and now all i get is a flashing question mark

Was this method from Apple Used ? Use Disk Utility to erase an Intel-based Mac

Then this would be used. How to reinstall macOS

Follow these guidelines during installation:

• If the installer asks to unlock your disk, enter the password you use to log in to your Mac.
• If the installer doesn't see your disk, or it says that it can't install on your computer or volume, you might need to erase your disk first.
• If the installer offers you the choice between installing on Macintosh HD or Macintosh HD - Data, choose Macintosh HD.

Follow the steps here: link-> If your Mac starts up to a question mark - Apple Support