Creating an Armorer Artificer

If I want to be able to create Armorer Artificer can I just buy Tasha's Cauldron of Everything, or do I also need Eberron also?

Hi Brian! Eberron contains: Alchemist Artillerist Battle Smith Armorer was added in Tasha's. So for that particular subclass, you would need Tasha's. Note that the other subclasses were re-printed in Tasha's, so if you were not interested in other Eberron content, Tasha's might be a better purchase..

Thank you, I appreciate it.

But unless I'm grossly mistaken, Roll20 doesn't support the Armorer Class features on the 5e OLG character sheet at all. There's no way to switch between the Guardian or Infiltrator Armor. It doesn't integrate your specialized weapons from the subclass. Also doesn't have a way to handle your Defensive Field or Dampening field. Just be prepared for a fair amount of manual input for this subclass.