If you have done all of the questions, you need to clear your test history before you can do the mock test questions again. To do this, go to the K53 Learner's Test while logged in and click "Clear All". This will clear your testing history and allow you to redo the test.
If you are unable to take the test and have not completed all the mock test questions, you could try updating your browser, or trying a different one. (Alternatives to Internet Explorer include Firefox, Chrome, Safari and Opera.)