Mana Shield is not an unlockable skill in Diablo 4
Sorcerer uses Mana as a resource, but there is no skill or passive called Mana Shield
If you mean a defensive Sorcerer skill, check your Skill Tree for Ice Armor, Flame Shield, or Teleport
If you mean a specific item, aspect, or seasonal power, verify the exact name in your Codex, inventory, or seasonal menu
If you want, I can help identify the closest Diablo 4 skill or effect you may be referring to