Can't backup IPhone to laptop

Hey, I'm getting really frustrated because ITunes is telling me that I don't have enough space on my laptop to backup my phone, when I know I do because I checked.

IPhone is an iphone6s 32GB that is currently maxed out (main problem I'm trying to fix when I ran into this problem), laptop is an hp laptop that runs windows 10 and I have itunes downloaded onto it. I looked in my C drive and I have 687GB of free space, so I have no idea why this is happening. I wanted to backup my phone so I could do a factory reset and maybe eliminate some of the "other" section that's taking up space on my phone.