Why can’t I install Windows 10? The disk is GPT but BIOS is legacy, need to convert to MBR,

I have a Lenovo IdeaPad Z570 laptop with Legacy BIOS (not UEFI). I’m trying to install Windows 10, but the installer says:

“Windows cannot be installed to this disk. The selected disk is of the GPT partition style.”

I tried using SystemRescue live OS and GParted. I deleted all partitions, but the disk still shows as GPT. I also tried sgdisk --zap-all, dd, and fdisk, but nothing seems to change it to MBR permanently.

I only have one USB and an external hard disk. I also tried using Ventoy to boot the Windows ISO.

My goal:

• Convert the disk to MBR.
• Install Windows 10 in legacy mode.
• Wipe everything if needed.