That's because somehow your lock icon was enabled for your render profile. All you have to do is toggle off the lock by clicking the small lock button in the upper right of the Output Tools window.
We introduced this concept of 'Render Profiles' into our 2017 SP0 release. This was you can easily create render profiles for commonly used Output settings, and then use the dropdown to switch between them for each project.
NOTE: you are by default always working in the 'Default Profile' render profile. Simply click the small "+" button to create and rename a new render profile.
Jeez... Sometimes you overlook the most obvious details. Thanks!