Yes you have to have stock recovery. You'd have to find a stock recovery.img file and flash it but depending on what version of the recovery file you get though you'd have to download a few of the updates.
If you want to update to nougat you'd have to update to latest version of marshmallow and go from there.
Any idea how I would go about getting the stock recovery.img file? Do I extract it out of the ROM? I know the process for flashing TWRP, but I've never flashed stock recovery back before.
I am currently on WW_126.96.36.199_20170629 (7.1.1) and I am trying to update to WW_188.8.131.52_20170816 (https://www.asus.com/ca-en/Phone/ZenFone-3-Laser-ZC551KL/HelpDesk_Download/).
So I would need get the stock recovery from the "WW_184.108.40.206_20170816"? Would I have to ask someone on WW_220.127.116.11_20170816 to send me a copy of the .img file or.... ?