Ｍy Closter cannot learn the "Razor Shell" moves from Move Relearner NPC.

 

Before the evolution, the Shellder can learn it through level up...

 

What can I do to re-learn the moves then?

Hello there @andrewchun. Sorry that this reply was late but I would like to inform you that the Cloyster you have now, learns the move "Razor Shell" from the Pre-Evo Move tutor located in the Day Care centres. Example being in Kanto Daycare the Pre-Evo tutor is located through the doorway behind the old couple, each Pre-Evo move will cost 10k to tutor your pokémon. To first access the day care the player must first be able to access Cerulean Cave and then defeat the Boss Named Naero, note defeating him on any level will allow the player to access the day care.

 

Regarding on where you can look up the learnset of a Pokémon, I advise checking out Pokémon Databse, this is an example for Cloyster's learnset, click here.

 

I hope this clears your doubts. Good luck and enjoy.