Offline conversion of system disks with earlier versions of Windows installed, such as Windows 7, 8, or 8.1 aren't officially supported. However, you must run the tool while booted into Windows 10 version 1703 or later, and perform an offline conversion. To resume BitLocker after conversion, you'll need to delete the existing protectors and recreate them.
You can't use the tool to convert non-system disks from MBR to GPT. Convert any attached MBR-formatted system disk to the GPT partition format.