I've recently created a magic item that has three modifiers, advantage on stealth checks, resistance to acid damage, and a natural weapon attack. However, once I added it to my character sheet and attuned to it, the natural weapon attack is not showing up in the actions tab. Is there a step that I'm missing? The item is currently published and called The Cloak of the Mimic based off a homebrew item I found online.

It is not currently possible to add an Action to a magic item.

okay, thank you

Try by creating it as a feat for that magic item.