Trying to reinstall macOS Big Sur after erasing the disk

Is your MacBook Air an Intel or M1 model?

Depending on which model, double-check how you erased the internal drive by using one of the following support articles:

For Intel: How to erase your Intel-based Mac - Apple Support

For M1: How to erase a Mac with Apple silicon - Apple Support

Then to reinstall macOS: How to reinstall macOS - Apple Support

Ok, well then you have the support article links to review. Let us know where didn't work for you or if you need further assistance with this issue.