Inside "Sample Test" block after declaration of create_chain_for_test method add these two lines:

start = create_chain_for_test(2, 5)
Test.assert_equals(loop_size(start),5)

That is enough to make test run.

You get that error because you can't run tests. The author has configured the tests to show you how the tail and loop are created.

That said i'm confused as heck about what the Node even has. It doesn't seem to have a value available with .value so how we determine loop from tail I have no idea