Legacy MBR boot is not able to recognize GUID Partition Table (GPT) disks. It requires an active partition and supporting BIOS to facilitate access to disk. OLD and limited on HDD size and number of partitions.

Does GPT work with BIOS?

Non-boot GPT disks are supported on BIOS-only systems. It is not necessary to boot from UEFI in order to utilize disks partitioned with the GPT partition scheme. Therefore you can take advantage of all of the features offered by GPT disks even though your motherboard only supports BIOS mode.

Is GPT legacy or UEFI?

GPT is modern and has many advantages over MBR. However, there are also some issues with GPT booting in Legacy BIOS mode. GPT is part of the EFI specification, of course it will work best in UEFI mode. But maybe it will not be compatible and can not boot up on the BIOS computer, see more here.