My iPhone 6 isn't syncing to my iTunes on my windows computer and "allow this device to access photos and videos" keeps popping onto my screen on my phone.

I go to connect my iPhone 6 to my windows computer and this pops up on my phone's screen: "Allow this device to access photos and videos? This device will be able to access photos and videos while it is connected to your iPhone." I click on allow and it still isn't syncing my phone. I don't know what to do and I have already uninstalled and reinstalled iTunes twice.