Unfortunately it’s one of very first steps encountered when one is first setting up a brand new “just out of the box” computer.
Setup asks if you want to save the File Vault Recovery Key to iCloud.
If the individual performing the setup selects “No” …
… the File Vault Recovery Key is then displayed on the screen and MUST be recorded for safekeeping THEN.
After that setup screen is cleared and FileVault encrypts the disk, there is no possible way to recover or display THAT particular key.
If the device password is then ALSO subsequently misplaced, the data in the disk is irretrievable.