ASP.NET Core Web API 500 Internal Server Error

I am trying to build an API within the ASP.NET Core framework. I made a basic controller that just returns a string:

namespace Dojo_Api.Controllers.Forum
{
    //[Authorize]
    [Route("api/[controller")]
    public class ForumController : Controller
    {
        private MainContext _context;

        public ForumController(MainContext context)
        {
            _context = context;
        }

        [HttpGet]
        public string Get()
        {
            return "string";
        }
    }
}

Now when I try to access the API via postman I get a 500 Internal Server Error:

I've tried changing the port, but it didn't work. Does anyone know a solution to this problem?

Thanks in advance!