I encountered this same problem and have been able to solve it. Filevault was used to lock the laptop. Previous employee left and we didn't have the password. When we tried recovery mode it still required a password, as did internet recovery mode.
The hint is in one of the other answers on here, which is that 'apple support says internet recovery doesn't require the login password'. This is actually true, it's just not readily apparent what to do after starting in internet recovery mode because the first screen you see is still the login screen. However if you click 'recovery assistant' on the top left, you'll see an option for 'erase mac' which will essentially erase the contents of the laptop allowing you to reinstall the macOS.
After erasing, the computer will restart and you'll see the the typical blinking folder with a question mark (because all the startup files were deleted), but you just follow the regular recovery process again (recovery mode by restarting and holding cmd+r) and you can reinstall the factory setting macOS.
I've run into other issues since restarting such as recovery images not unmounting properly so I'll probably need to use a bootable USB, but at least the HD is wiped and finally unlocked!