The syntax error you are encountering when trying to unlock BitLocker using the command prompt may be due to the incorrect use of the -rp switch. The correct command format for unlocking a BitLocker-protected drive using a recovery password should be as follows:
manage-bde -unlock C: -RecoveryPassword <48-digit recovery password>
Make sure to replace <48-digit recovery password> with the actual recovery password without the angle brackets. Ensure that there are no extra spaces or characters in the command, as that can also lead to syntax errors.
If you continue to face issues, double-check the recovery password for accuracy and ensure that you are running the command prompt with administrative privileges.