"The target personal computer is locked by BitLocker encryption and requires a 48-digit Recovery Key to boot into Windows."
Without the correct recovery key, the computer cannot be unlocked. There is nothing that you, or we, or Microsoft can do to change that.
The required recovery key is generated towards the end of Windows installation and automatically stored in the Microsoft account that was used to install Windows. If you no longer have access to that Microsoft account, and if no one made a backup copy of the recovery key, I'm very sorry but there is no chance for the computer to be unlocked.
The computer is not useless - you can erase the hard drive and reinstall Windows, which will generate a new BitLocker recovery for that installation, and continue to use the computer. But erasing the hard drive erases everything that was previously on the computer.
BitLocker was created to protect the privacy and security of a computer owner in the event the computer is stolen. As a result, it is a complete dead end. If there was some way around it, attackers would eventually figure it out.