Where is my project.json?

I'm building a small asp.net core 2.0 with razor pages. I want to target framework 4.6.1 to use some libraries. All examples in google point to modify the project.json but I can't find this file in my project/solution. 

I tried right clicking on my project and just change the target framework with the gui but only core 1,core 1.1 and core 2 are the only available selections in the dropdown. 

Any ideas?