"What is with the lamp?"
Translation:Was ist mit der Lampe?
After the preposition "mit", you have to use the dative case. That's why "die Lampe" (feminine nominative) becomes "mit der Lampe" (feminine dative). So the sentence Duolingo gave you is fine, whereas *"Was ist mit die Lampe?" is wrong.
Just to clarify, this phrase means the same in German as it does in English, right? "What is (up) with / wrong with the lamp"?