There is a minor typo in the /shop in regard to prismarine crystals. The command /find prismarinecrystals does not return any results, and the command /find prisemarinecrystals (note the extra 'e') maps to the shop item instead. This is a common mistake, but the correct version of the word can be verified on the Minecraft forums. (Note that the commands /find 410, /find prismarineshard, and /find prismarine work as expected.)