So, I hope this doesn’t come off as rude, I’m genuinely trying to help you out.

You speak English very well, for starters.

The problem with your post is

“People who have never been in a relationship before” means just that: someone who has never dated anyone.

Starting the sentence that way leads us to believe that you are asking the question to only people who have NEVER dated ANYONE.

The question you asked is “what did you wish you knew about relationships before getting into one?”

This question implies that the person you’re asking is somebody who has been in at least one relationship.

Do you see where the problem is?

A correct version if you’re asking someone who has been in a relationship would be “people who have been in a relationship, what do you wish you would have known before getting into one?”

Like I said, your English is great and I hope this doesn’t come off as rude, I just want to help.