In surah Al-Ma’idah ayat 96, it explains that it is lawful for us to eat seafood, so why we can’t eat fish without scales and other sea creatures like clams, lobsters? Would you mind give me the strong narration about this?

When my sunni friends know about shia not eating lobster, clams, they are wondering because what they know is Al quran allows us to eat all creatures that lives in the sea

moreover, I read a narration that said that we can eat all sea creatures even the carcasses meanwhile in shia it is forbidden to eat the animals that are dead in the sea, why is it contradictory?

Because we have so many narrations that quantify what the Quran says. We cannot abandon Hadith. Many Hadith say fish must have scales. You can find this in Alkafi, wasail alshia etc