In 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?

The Qur’an gives us general guidelines. It does not give us specifics. We tale the specifics from hadith. For example, does the Qur’an tell us that eating dogs is haram? It doesn’t but all Muslims agree that it is haram.

Here’s one hadith that states this:

In a hadith from Imam Baqir (A.S.), Muhammad ibn Muslim asks:

"We are given fish that do not bear an extra layer on them (scales)". The Imam replied: "Eat all fish that have scales and do not eat those that do not."

The only seafoods that are permissible are:
Fish with scales
Shrimp (prawn)