The base phrase
etw. darf nicht sein
(often in the form of) Das darf doch nicht sein!
literally means that something is not allowed to be/must not be (following some set of rules), but is almost exclusively used for things that are happening, inferring the disfavor of the speaker: "I can't believe this is happening [since it's not allowed], right?"
etw. dürfte nicht sein
on the other hand, expresses that something is happening and that it is apparently being tolerated (implicitly allowing it), which - following the writers opinion - shouldn't be. Similarly to how you need this subjunctive in English, you need the dürfte in the text (Konjunktiv II / Irrealis).
A literal translation would look like this:
A debate that shouldn't be one.
I'm not sure that can be said in English, so here are some other tries:
A debate, that shouldn't have become one in the first place.
...initiated a debate, where there shouldn't be one.
...now there's a debate, where there shouldn't have been one in the first place.