Macbook air 100 GB hidden space + error when verifying volume
Forget the space calculations if your disk has accounting errors. 


Back up any data that’s not already safe. 
Try repairs until you’re out of patience or feel it’s not going to work
Erase the drive (Apple support erase install guide is below - search for erase or format)
Re-run the disk check. 
If you have a clean check, then validate the storage settings before you load any data. Erase again if you aren’t convinced the clean install is correct. 
Seek hardware service or repair if the check won’t pass after two erases. 


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