If it is a kid, then it means "to behave in a spoilt manner" 如果孩子得不到他要的東西, 他就撒娇了 = if the kid didn't get what he wants, he would /throw a tantrum/behave in a spoilt manner. 娇 = 可愛
If is a girlfriend, 如果她得不到她要的東西, 她就撒娇了,
1. If she does not get what she wanted, she would act coquettishly
2. .... she would become coy/kittenish to get what she wanted.
娇 = 媚誘