Back up encryption for iphone 11 pro max

Hey, i need some help.

I just plugged my phone into the USB port to my laptop so I could sync my music onto it. the itunes app on my laptop asked if i wanted to turn on the backup encryption and my dumb arse said no... come to find out, i do in fact, need it on for other reasons. however, when i went to try to turn it on it didnt give me the option to. I'm confused and certainly do not want to have to call apple support just for this.

please help,

not tech-savy