system is set to Legacy BIOS mode, but Linux unalocated partition is efi so it refuses to install Windows
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If your system is set to Legacy BIOS mode but the Linux installation has an EFI partition, you may encounter issues when trying to install Windows. In Legacy mode, Windows typically expects a Master Boot Record (MBR) partitioning scheme, while EFI requires a GUID Partition Table (GPT).

To resolve this, you can convert the GPT disk to MBR, but please be aware that this process will erase all data on the disk. Ensure you have backups of important data before proceeding.

Here are the general steps:

    Backup Your Data:
        Ensure that you have backups of any important data on your Linux system since converting the partition table will result in data loss.

    Boot from a Linux Live USB:
        Create a bootable Linux Live USB (such as a Linux Mint Live USB).
        Boot your computer from the Live USB.

    Open GParted:
        Use GParted to manage partitions. If it's not installed, you can install it using the package manager:

        bash
        sudo apt-get update
        sudo apt-get install gparted

        Open GParted from the Live environment.

    Convert GPT to MBR:
        In GParted, select the disk containing the Linux installation.
        If there's an EFI partition, delete it. This will erase the Linux installation, so ensure you have backups.
        Click on Device in the menu and choose Create Partition Table.
        Choose msdos as the partition table type (this is MBR).

    Create a New Partition for Linux:
        Create a new partition for your Linux installation. Format it as ext4 or your preferred file system.

    Install Windows:
        Now that the disk is MBR, you should be able to install Windows on the unallocated space without EFI-related issues.

    Reinstall GRUB:
        After installing Windows, you might need to reinstall GRUB to have a dual-boot system.
        Follow the steps mentioned earlier under "Reinstall GRUB (if necessary)."

    Verify Boot Mode:
        After completing the installation, verify that your system is set to Legacy mode in the BIOS/UEFI settings.

Remember, this process involves data loss, so ensure you have backups and are willing to reinstall both Linux and Windows. Additionally, the steps might vary slightly depending on your specific Linux distribution and hardware, so consult relevant documentation as needed.