I cannot use discards in c#

Question

According to this link discards are supported since c# version 7.0 https://docs.microsoft.com/en-us/dotnet/csharp/fundamentals/functional/discards

If I write an #error version it says 7.3

This code in the same MVC app doesn't work:

        public ActionResult SomeEndpoint()
        {
            return SomeResult(out _);
        }

        private ActionResult SomeResult(out int theInt)
        {
            theInt = 42;
            return View();
        }

The error is: Error CS0103 The name '_' does not exist in the current context

What am I doing wrong?

Update: Is the target framework the problem? It's only 4.5.2.

I'm using visual studio 2019