Some regions make that function illegal, so it's not possible. Also some devices, not sure what phone you have and what software it's running. I assume you've checked in settings and it's all off...
My S6 and S7 both remain silent if i have the mute on..
Ensure that you do not have, "Easy mode", enabled in your phone's settings, (some camera functions are not shown in this phone mode), and then follow the instructions in the video... (apologies for the language contained in it)
I am in Canada and up until 6.0.1 the shutter sound was something that was baked in and could not be disabled. Now, the opposite is true. I have NO shutter sound (regardless of my phone's volume - also not rooted) and there is no control in the camera settings to even enable it if I wanted to. I personally like having no shutter sound because it lets me take candid shots of my pets or kids without them hearing the "CLICK" and stopping whatever they were doing to look at me.