Translation:If there is no discount, I won't buy it.
The way 有/没有 were taught to me in school, they denote existence of something or more abstractly the occurence of something. 是/不是 is about equivalence, in contrast. So 我是一个老师 is I am (equivalent to) a teacher. And 我有一个老师 states that there exists one teacher associated with me. Without a subject to "have" the existence of the word after 有, it's just a statement of existence (and/or the subject is implied). So here 没有打折 means there exists no discount/there has not been created a discount/a discounting action has not occured.