Download and install the latest Aircrack-ng development (beta). There's an --ignore-negative-one switch that you can use to fix that fixed channel -1 annoying error.
Here's how you can install the latest Aircrack-ng.
Type the following commands in a terminal (Ctrl+Alt+T).
sudo apt-get install subversion libnl-dev
cd /opt/
sudo svn co http://svn.aircrack-ng.org/trunk/ aircrack-ng
cd aircrack-ng
sudo make
sudo make install
Once it's done, run aircrack-ng without any options, and look at the first line, it should read something like this:
Now, when you use airodump-ng and aireplay-ng, always use the option --ignore-negative-one.
