Problem with controllers and routing using asp.net core

Im new to asp.net Core and i’ve stumbled upon problem with my routing/Controller. Not sure which one is faulty. Overall I don’t know how to set up multiple controllers for multiple .cshtml files which will allow me to move between pages or do simple CRUD operations. I’ve created standard Web application project. My Project schema looks like on picture before.

Visual Studio 2019 build errors

I have API project (.NET Framework 4.6) that contains a lot of different controllers, nuget packages, swagger, EF etc.
If i load this project & make a build – everything succeeded (No issue found) and i can deploy my project to DevOps. However if i add atliest one line of code & make a build i get more that 6000 errors.
I already tried to clean & rebuild my project but this does not help.