I just created new Web API project (using MVC) in visual studio 2015 and for the testing purpose, I ran that project but ended up below error.
After running the project,
In the ValuesController there is an attribute Authorize if you remove it, then it will work as home page.
The Authorize attribute just prevent an anonymous users from accessing the ValuesController.
to work with this attribute, you need first to register a user, and then login to get user's token, then you can use the token to authorize your self and get access .
In this page Individual-accounts-in-web-api is explained all what do you need