Mac OS X, unable to un/mount Macintosh, password appears

I was trying to reinstall OS on my old Macbook Air 2015. Then, I erased all the data from it and something happened Macintosh HD get greyed out, and get encrypted. I tried many other solutions on Internet, but I still has a problem. It just can't get unmounted, uncrypted or something else. When I trying to reinstall Mac OS El Capitan it asks about password on Macintosh. I did not set any passwords or other stuff. And everytime when I click cmd + r it opens in Internet Recovery(cos I wipe the disk). Any ideas? Thanks in advance.