I think it's perfectly okay not to understand anything on a math placement exam. In fact, sometimes placement exams are designed to be difficult. They're literally trying to see if you know enough material to skip one or more math courses, so it's no big deal if you do poorly IMO. They just want to make sure they're not wasting a student's time by teaching them stuff they already know. Plenty of students "fail" the placement exams and it's fine/normal.

You'll eventually learn everything that's on the placement exam. I know math can look like a foreign language sometimes, but universities and community colleges typically offer free tutoring for students who need added help. You just have to find out through your campus' resource center (or student services center) on how to get a tutor. Typically, the tutors are other students who've already taken the course and are getting paid an hourly rate by the college/university to help out other students. And believe me, there is nothing wrong with seeing a tutor. I saw a few when I was in undergrad and I found them incredibly helpful.