Do you think you can clarify what you mean by, "did not get chemistry"? If you mean you just don't understand it, that's okay, you still have lots of time to work on it - you're only in year 10.
If you mean the school won't let you take it, I'm sure something can be done to put you into the class, but you need to figure out why they didn't let you. It could be as simple as there was no timetable that could be made with your selection of subjects - this might mean having to change one of your subjects so you can do chemistry. If you talk to the school, and let them know the situation, I'm sure something can be done to get you into chemistry. And if worst comes to worst, you can always enrol via distance.
Also, though, it's worth noting that what you CAN take chemistry 3/4 without taking chemistry 1/2. Yes, it will be hard - but I've seen you on the forums, you're dedicated and hard-working. I'm sure you can make it work - potentially even by studying chemistry 1/2 on the side so that you don't fall behind, and will be ready to take 3/4 (though if you take this approach, I highly encourage you to find out what topics are relevant to 3/4 and which aren't, so you don't waste your time)