There is no
_fixed_process-function in Godot 3.X anymore, use
_physics_process! Also make sure that you defined an InputAction named "MoveUp" or use "ui_up" (that will be available by default). Lastly, be aware that in Godot the positive y-axis points down, not up (like you may be used to from Mathematics). So currently your character will move *down* when you press "MoveUp", not up (as the name suggests).
Also when you post a question, please make sure that your code is formatted properly (each line has to start with 4 spaces, each additional 4 spaces equal an indentation layer). If you don't, underscores are interpreted as markdown and indentation is lost, which makes your script a lot harder to read for others trying to help you.