Tried factory resetting my Macbook Air but won't erase disk
Try erasing the entire drive, not just the volumes.
In Disk Utility, Hit  Cmd ⌘   2   (or see the View menu) to show Devices instead of just Volumes.  Select the overall drive you will now see over the volumes (now in a disclosure triangle instead of separate) and Erase that, GUID/APFS.
If you aren’t following an internet recovery factory erase, here is the best one:

https://support.apple.com/en-us/HT204904

Click through to the erase portion.
Selecting the correct item in Disk Utility almost always gets an erase completed without reverting to the terminal app and command-line tools.  Also, Apple Support is now delivered almost entirely without Genius Bar visits.