It should be:
couatl.icaoprefixes = SB
If you put the ? it means that vehicle repaint will have a chance to appear anywhere, which is not what you wanted, if you wanted it only in brazilian airports.
However, if you still can't see your repaint, check what airport you are in. If there are other repaints with an higher score for that airport, your vehicle will never "win" against them. For example, the "Gol" repaint that comes with GSX, has a score of 4 at SBSP, SBEG and SBCT, and the "Tam" repaint has a score of 4 at SBFL, which means you they will always win against your repaint in those airports.
If you want to give your vehicle a chance to appear at, let's say, SBSP, you need to have both SB and SBSP listed, like this:
couatl.icaoprefixes = SB SBSP
This means your repaint will score 2 in any airport in brazil, and 4 at SBSP. When the score is a tie, the selection is random.