Learn Hardcore Croatian: Poništite pogrešno pitanje.

Poništite is the imperative form of the verb poništiti. In other words, it is a command: cancel, invalidate, annul, or undo.

So the sentence is giving an instruction to do something to pogrešno pitanje.

Why does poništite end in -te?

The ending -te is used for:

a command to more than one person
a polite/formal command to one person

So Poništite can mean:

Cancel ... when speaking politely to one person
Cancel ... when speaking to several people

If you were speaking informally to just one person, you would usually say Poništi.

Could poništite also be a present-tense form, not just a command?

Yes. The form poništite can also be the 2nd person plural present form of poništiti.

However, in a short standalone sentence like this, especially as an instruction, it is naturally understood as an imperative.

This is very common in Croatian: the form itself can match another tense, but the context makes the meaning clear.