The secure boot settings is one thing. Also you need to have only the m.2 drive installed. No other hdd while installing. This is also a uefi bios if you are not familiar. It can be done.
Not much settings in these bios. not much you could screw up.
Go into advanced settings in bios. Then secure boot Then key management. Save the keys if you like then clear secure boot keys. Go back and make sure it says secure boot disabled. You could try that but I have always had to remove the other drive in the past. To do that you have to unscrew and open it up. I would try and get Linux installed with uefi, you would use rufus or something for that.
As far as opening and warranty I can't answer that I don't work for Asus. Be careful though the little plastic tabs can beak easy, It doesn't really matter if they do though the screws hold it.
@Daniel_zabojca: did you get anywhere with linux on this laptop model?