The test can be taken twice. We advise that you take one test at the very outset of your learning, so that you can see what level you currently are. Then, having spent some time working through most/all of the learning materials, you may want to take the second test to see how much you have improved by. Very few other courses give you an objective and scientific means by which to measure your learning. Here's how to get your certificate after completing your test.
Not all courses have a test. If you don't see the link on your dashboard you can assume yours doesn't.