Select the node you've named GUI, bring the mouse over to the viewport header where it says Layout, click it and pick the Full Rect item from there.
Now try the expand Size Flag on the Bars node again.
If you find that you don't want the vertical expansion on the GUI node select it again and in the inspector dial the Bottom anchor value to 0. Or alternatively pick the Counters node and disable the Vertical Fill Size Flag on it. Actually I would go for disabling the Fill Size Flags either on the node named Counter or Background in there.