The program required to perform the function you selected is either not installed, or is installed in a folder other than the one specified in the Program Locations configuration dialog (choose Configuration from the Options menu, click the Program Locations tab).
For a complete list of supported external programs and information on obtaining and installing these programs see System Requirements and Installing Optional External Programs.
For example, this problem can occur if you try to extract files from an ARC file, but have not specified an ARC extraction program in the Program Locations configuration dialog.