Macbook air

If you have a model from Late-2011 or later:

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

1. Restart the computer. Immediately, at or before the chime, hold down the (Command-Option-Shift-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 target drive (out-dented entry w/type and size) from the side list.
5. Click on the Erase button in Disk Utility's toolbar. A panel will drop down.
6. Set the partition scheme to GUID.
7. Set the Format type to APFS (SSDs, only if installing Mojave or Catalina ) 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 Install OS X and click on the Continue button.

I'm sorry but you need to be clearer and provide some detail. I don't know what you did. Why were you choosing a startup disk? What bootable USB?

Please read Writing an effective Apple Support Communities question. We are here to help. We are all users. We are not mind-readers nor clairvoyants. We only know what you write down and tell us. Be specific and thorough. Include your hardware information and installed version of macOS. Try to write clearly. Think about what you want to say before you write it down, then re-read it to see if it makes sense.

I think I need to know your exact Mac model. To find the complete model information select About This Mac from the Apple menu. You can create a snapshot of the dialog as follows:

How to take a screenshot on your Mac

Capture, Save or Record Screenshots in Mac OS X - Help Desk Geek

OS X Screen Capture Shortcuts

(⌘⬆︎3) Command-Shift-3 Capture the screen to a file

(⌘⬆︎⌃3) Command-Shift-Control-3 Capture the screen to the Clipboard

(⌘⬆︎4) Command-Shift-4 Capture a selection to a file

(⌘⬆︎⌃4) Command-Shift-Control-4 Capture a selection to the Clipboard

(⌘⬆︎5) Command-Shift-5 Invoke new screen grabber

You can then post the image in the forum's message editor.

Macs prior to Late-2011 cannot do an Internet Recovery. If you completely erased your disk, then you also erased the Recovery HD. If you only erased the startup volume, usually called Macintosh HD, the Recovery HD may still be intact. Your OS can be restored using Recovery Mode - restarting with Command-R. If you cannot use Recovery Mode, then your only other options are Internet Recovery, if your model supports it, or restore from external bootable media such as the DVD that came with the computer or a bootable USB flash drive. So, I need to know more about what we are dealing with in terms of your hardware and version of macOS that was installed before you erase the disk. I assume that was 10.13.6 High Sierra.

Did your MBA come with a recovery USB stick? If it did, then use that to restore the factory system. If it came with Software Restore DVDs, then you need a USB optical drive to restore the factory system. You can upgrade from there.

The only other alternative, I believe, is the following:

You can purchase Lion at the Online Apple Store. The cost is $19.99 (as it was before) plus tax.  It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.