Rotobrush not masking

You are applying RB to a still image, which of course won't work, given that it is based on detecting motion in clips. You may want to read the online help and find a good tutorial on the basics.

Mylenium

Cropped screenshots are almost useless for diagnosing problems, and uploaded ones are a pain to look at. Try embedding a full screenshot showing the timeline, the modified properties of the problem layer, and the footage panel. Select the layer, press 'uu' to reveal modified properties, take a screenshot, copy the file or drag it to the reply field or use the Toolbar to embed it so we can see it. 

The only thing I got from your screenshot is that you are trying to use Rotobrush on a JPG. That is the wrong tool to use on an image. Rotobrush is intended for Footage.

You could get a white line if you selected the wrong display for Rotobrush in the Footage panel.

Not all Footage is compatible with Rotobrush. There must be significantly different color and luminance values between the subject and the background for Rotobrush to work. You cannot rotobrush a farmer in a yellow shirt standing in a wheat field or a man in a black suit standing in front of a black car because there are no distinct edges for Rotobrush to find. Motion blur is also a big enemy of Rotobrush. I have had 100 frame shots that took me a couple of days to roto. I have also had 10-second shots that Rotobrush could successfully process in just a few minutes.

Make sure you have looked at some good Rotobrush tutorials, make sure your Footage is well suited to the process, and embed a full screenshot if you have more trouble. It is also a good idea to include a detailed description of your design goals in the post. 

Have you actualyl selected the right tool? Screenshots might help to clarify.

Mylenium