Starting up from macOS Recovery
To start from macOS Recovery with FileVault enabled, start up your Mac with the Command and R keys held down, then release when you see the Apple logo. The Recovery System opens normally and you can perform any of the tasks that are listed.
To mount, verify, or repair a drive or volume that is encrypted with FileVault using Disk Utility, enter the FileVault password when prompted.
Some startup key combinations, such as those that use the Option or Command keys, might not be recognized if a firmware password has also been enabled. Firmware (EFI) passwords not the same as FileVault passwords.
If you have a firmware password enabled on your Mac, you must successfully enter that password before you can start from another volume, or before you can access the Startup Manager.