problems with the Roto brush tool

Rotobrush is the wrong tool to create a matte when you have a plain background. You should be creating a procedural matte by manipulating the color values on a duplicate copy of the footage that you can use as a track matte. For that footage, duplicate the footage layer, rename the duplicate layer "Matte," add the Black and White Effect with maximum negative values for Red, Yellow, Green, and Magenta, then set the layer as an Inverted Luma Matte for the original footage layer should do the trick. A simple shape layer background for a sky and a couple of clicks will get you there:

I also have a suggestion for your posts. When you want to share a screenshot, you can copy and paste them or drag them to the reply field instead of using the Drag & drop here... button. You can also use the toolbar to place images and videos directly in your post. It makes them a lot easier to see. The Drag & drop here... button is useful for sharing AEP files if you need to do that.