Make boot partition using GParted in Boot-Repair [closed]
You should be able to right-click on the partition you want to set as bootable and click "Manage Flags", and then check the box for the boot flag.
If you are using parted, you can print for the list of partitions, and then set 1 boot on to enable the boot flag for the first partition. If your drive uses a GPT partition table, you'll want to create a bios_grub partition instead using set 1 bios_grub on.