Make sure you have John installed on your N900. Use the following command to check:

which john

If you come up empty try the following:

sudo which john

If you still come up empty, you need to install john:

sudo apt-get install john

However, I think this is now broken with the latest Maemo version (1.7.8) of John, since I can't make it work on my unit anymore either. With this latest version we can't use the option to use a configuration file anymore, which is what the script in FAS relies on.

I will have a look at this to see how we need to fix this. This is probably related to the fact that the latest Maemo version of John is the plain vanilla version, not the community enhanced one.

Edit: Other reasons for quitting include no handshake was captured or your cap file is empty.