This key is usually saved to your Microsoft account, a USB drive, or printed when BitLocker was first enabled. Since you've already checked your account and it's not showing there, a few more things to try:
Sign in to https://account.microsoft.com/devices/recoverykey with any other Microsoft accounts you may have used on this device, including a work/school account or a family member's account who may have set it up
Check if you have a printed copy or a USB drive with a saved .txt file containing the key.
If you used a work or school account, you may need to contact your organization’s IT administrator.
Unfortunately, if the recovery key is not available, it won’t be possible to access the system because of BitLocker encryption. In this case, the only option is to format the hard drive and reinstall Windows.