The meaning of "makuha sa" is practically the same as the actual translation of the 2 words. "To get by means of" (to get the desired result) is its translation. "kung hindi makuha sa ibang gamot nA pang-ubo, uminom na ng sirUp na Vicks" = if the desired result can't be achieved through other cough medicines, Vicks syrup should already be taken". That would be the more direct translation of your sentence, though of course, it means the same thing as your translation.

kuha is to get. makuha too, is literally to get Hindi makuha sa is If you can't get it (done) by doing this or that. makuha is to get something done or accomplished or solved or finished or covered or over and done with or defeated