If you are unsure about whether or not you need to retake a test, it is always best to contact your instructor. They can provide you with information about their retake policy and the steps you need to take if you need to retake a test.
In general, Plato courses are designed to be self-paced, so you can work through the material at your own pace. If you do need to retake a test, you will have the opportunity to review the material and prepare yourself before taking the test again.