Note that errors are in red text and warnings in yellow. There is no issue with ignoring warnings, the game will still run, but it may be a good idea to fix them out of habit, that way you are more likely to see important warnings and just as a good practice anyhow since some warning may be helpful. In this case it is telling you that the function returns a value that you are not using, so just make a new value like this:
var scene_val = get_tree().change_scene(mapScene)
