Hi Mohamed,
To start, while leaving the last question unanswered is certainly worse than simply answering it wrong, it was only 1 question (meaning that the 'penalty' from not answering it would have been minimal: no more than 0 - 10 points off of your Overall Score).
If you have taken just this one practice CAT/mock, then it is difficult to say what your actual 'ability level' is right now (as you may have gotten a bit 'lucky' or 'unlucky' when you took this Test). That having been said, the difference between a 550 and a 410 is significant enough that it would be unlikely that you could improve by 140 points in just 10 days. While there's no harm in taking the Official GMAT as scheduled, you could potentially save some time, money and effort by paying the rescheduling fee and pushing back this Test Date.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) What were the Quant and Verbal Scaled Scores for this CAT/mock?
Goals:
4) Is your overall goal score 550 or something else?
5) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich