The Circle tool only draws an unfilled circle but with a bit of lateral thinking, it can be used to create a solid circular area.
- To create a circular area of diameter X, draw a circle and then set the Radius to X/4;
- Set the Width to exactly X/2;
- Check the area if the PCB units are swapped.
- That will create a circular area with no hole in the middle.
Note that a smaller (or larger!) Width will leave a hole in the middle.
Brand new interactions and interfaces. 
Easy to use and quick to get started. 
And you can try:
Method 1:
1, Draw a circle
2, Right-click it , and use "Convert to Board Cutout"
3, Click the Converted object(which is a solid region in multi-layer), change its type to "solid", layer to other layer, such as topsilklayer
Done.
Method 2:
1, Use "Solid Region" tool
2, Press hotkey "L" to change routing shape to Arc
3, You will draw a half of circle in one step, two times become a circle.