How can I solve "fixed channel -1" (mon0 is on channel -1) issue when using airodump-ng?

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).

1. sudo apt-get install subversion libnl-dev
2. cd /opt/
3. sudo svn co http://svn.aircrack-ng.org/trunk/ aircrack-ng
4. cd aircrack-ng
5. sudo make
6. 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:

Aircrack-ng 1.2 beta1 r2312 - (C) 2006-2013 Thomas d'Otreppe

Now, when you use airodump-ng and aireplay-ng, always use the option --ignore-negative-one.