I also own MT6737m with Android 6 and getting Android 7 or 8 to work has been a tough experience. And of course yea, you need to unlock your bootloader. Then get a tool to unpack your stock boot image head on to fstab.6735 then delete verify on the /system mount flags. Repack your image. Then delete the verity_key from your /system partition and you should be good to go. Right now am on my free time I can patch up the boot image for you ONLY if you can trust a stranger! Haha...