Generally speaking, the defense is twice, once when the subject report is opened and once when the thesis is defended.

The first defense is the defense of the opening report. If you fail, you must re-select the topic and re-write the opening report.

The second defense is a thesis defense, which is very important. If the defense fails, there will only be a second trial. You have to re-select the topic and re-write the paper, which is very troublesome. So you'd better pay attention.