It wont let me do it
When I choose to reinstall macos, the only disk option is gray, it wont let me choose it. It’s Macinstosh Hd
MacBook Air 13", macOS 10.14
Boot into Recovery (Command R) and from the dropdown menu: Utilities> Disk Utility> run the First Aid on your Macintosh HD (and the "Macintosh HD-Data" volume as well if Catalina) If errors are found and repaired, run again until no errors reported.
Recovery: https://support.apple.com/en-us/HT204904