Getting [The currently executing thread is not in a step definition context] error

BDD style unit test for my ASP.NET Core 3 API is erroring. Packages referenced (xunit:2.4.1, xbehave:2.4.1,xunit.runner.visualstudio:2.4.2).

Here is a simplified sample test that results in the same error:

        [Fact]
        public void Test1()
        {
            int x=0, 
                y=0, 
                sum = 0;

            "Given x = 20".x(() => x = 20);
            "And y = 10".x(() => y = 10);
            "When adding x & y".x(() => sum = x + y);
            "Then 30 is returned".x(() => Assert.Equal(30, sum));
        }
    }

And running the test throw exception:

Message: System.InvalidOperationException : The currently executing thread is not in a step definition context. Stack Trace: CurrentThread.Add(IStepDefinition item) StringExtensions.x(String text, Action body) UnitTest1.Test1() line 15