Hello,
Thank you for using Microsoft products and posting to the community.
Do you remember the password or recovery key to the BitLocker-encrypted partition?
If you do, since you have mentioned that you tried "manage-bde", please check whether you have entered the command correctly.
To unlock the drive using your password, please enter:
manage-bde -unlock C: -password
To unlock the C drive using the recovery key, please enter:
manage-bde -unlock C: -recoverypassword (48-digit key including dashes)
After unlocking, turn BitLocker off so you can access the drive normally:
manage-bde –off C:
If the error still appears, please provide a screenshot so we can understand your situation.
Please feel free to let me know if you have any further updates, thanks.
Best Regards,
Johann - MSFT | Microsoft Community Support Specialist