Both phrases are correct and their meanings are the same. Which one to use ends up being a matter of personal preference. For practical purposes, if you want to use one and are trying to decide which one to use, pick the one that is the easiest and most comfortable for you to say. ex) an employee wants to make a lateral transfer to a different department at work. The manager signs off on the paperwork to consent to the transfer but tells the employee that the transfer involves a pay cut! The employee is upset. He shakes his fist in the air and yells "this shouldn't have to happen! It's the same job but on a different floor! Why do I have to take a pay cut! This shouldn't happen to me-- I've worked so hard!!!" ;)

Both phrases can be correct, but they have slightly different meanings and uses: "This shouldn’t happen": This phrase implies that the event or situation in question is undesirable or unexpected. It suggests that the event is not appropriate or acceptable. For example, "This shouldn’t happen in a well-run organization" means that it is inappropriate or not expected for the event to occur. "This shouldn’t have to happen": This phrase implies that the event or situation is unfortunate or unnecessary, often suggesting that something could have been done to prevent it. It conveys a sense of inevitability or regret. For example, "This shouldn’t have to happen if we had planned better" suggests that the situation could have been avoided with better planning. In summary, use "this shouldn’t happen" to express that something is not desirable or appropriate, and use "this shouldn’t have to happen" to express that something is regrettable or preventable.