What you did was create a variable of a ThirdPersonCharacter type, but it has no idea what object in your scene it’s referencing.
Delete the “As BP Third Person Character” and connect the “BP Third Person Character” to the cast instead. You do not need 2 variables of the same type - unless you have 2 “Third Person Characters” in your scene you want to keep reference to, of course.
You are getting the reference for this variable by using “Get Player Character”, then casting to the type of character you want it to be, to confirm it is what you expect, and on success you are then setting the variable with your reference.