At a glimpse, you seem to be accessing a variable that saves a reference to your ThirdPersonCharacter… Inside of the ThirdPersonCharacter.
You can just edit “Current Potion” and “Current Vit” directly from the character’s blueprint, because those variables are its own. Referencing the owner is only necessary inside of the Pickup blueprint.
As for the error message, you are reading your character reference from the variable “character” inside of ThirdPersonCharacter. Does that variable actually contain any data? You might have forgotten to set it up.
In fact, as you are using that variable to access all of the data (Potions and Health), this error messages might be the reason why nothing works.