if this is external to the respective Pawn then it would be MyCharacter->GetMovementComponent()->IsFalling()
the GetMovementComponent() returns a UPawnMovementComponent* so you use the arrow operator not the dot operator (unless you are going to store it as a Stack reference, but that generally is not “safe”)
otherwise what is the error you are getting when you hover over the MyCharacter in your IDE, or in the output window about that line?