As you are adding abilities or items you may hit an error similar to the following:
Error: Unable to transition to LayerName.StateName because the state doesn't exist.
Where LayerName is the name of the Animator Controller layer and StateName is the name of a state which doesn't exist. In this example we are going to fix the following error:
Error: Unable to transition to Base Layer.Climb.Ladder Bottom Mount within because the state doesn't exist.
Although this error relates to an ability the same concept applies to items. You will hit this error if the ability or item is trying to play a state that doesn't exist in the Animator Controller. For this example it is trying to play the Climb.Ladder Bottom Mount state within the base layer (layer 0). I can fix this by creating a new state or by copying the state from an existing controller. In this example I only need to copy the climb substate machine from the Adventure controller.
<- Disable Input