OK, so that means the cfg file containing the alias "quickknife" is not being read.
Search your system for saved.cfg. autoexec.cfg should go in the same folder. You may have 2 saved.cfg files (many Windows installations do).
If you find 2 saved.cfg files, one of them will be sharing its folder with the autoexec.cfg you've been editing. The other will share its folder with a (mostly) empty autoexec.cfg file. that is the autoexec you should be editing.
Also, just in case, open up assaultcube.sh (if that's what you use to run the game) and post its contents here.
Search your system for saved.cfg. autoexec.cfg should go in the same folder. You may have 2 saved.cfg files (many Windows installations do).
If you find 2 saved.cfg files, one of them will be sharing its folder with the autoexec.cfg you've been editing. The other will share its folder with a (mostly) empty autoexec.cfg file. that is the autoexec you should be editing.
Also, just in case, open up assaultcube.sh (if that's what you use to run the game) and post its contents here.