The Bezier curve is not closed. In this case, it is still a line, not a surface, so it cannot be filled with color.

You can choose to select the two connection points, the head and the tail, and click the welding tool in the upper left corner of the toolbar to close the curve. As shown in the picture:

Another method is to use the graphics tools on the left vertical toolbar (rectangle, ellipse, and basic shape tools are all available) to directly draw the shape and then convert it into a curve, and then Use nodes to adjust the shape.