Reinstall the aircrack-ng suite and it'll fix it. This issue has been around for awhile now. I'm not sure what makes it happen but hopefully it'll be fixed just whenever the next release comes out.
Here's the instructions that I wrote for someone else with the same problem awhile back.
Uninstall the aircrack-ng suite (airmon-ng is included in this) with this command:
Then download the sources from the aircrack-ng website here:
apt-get autoremove aircrack-ng
Extract the .tar file with this command:
You will need to update some of the SSL-dev files so run this before building from source:
tar -xf aircrack-ng-1.1.tar.gz
Enter the directory that was just extracted and then run:
apt-get install libssl-dev
Reboot and everything should be good to go now.