there were no valid, saved preferences available.  Use the control panel to choose your settings and restart your computer to solve this problem.

a vital resource could not be allocated - most probably there was not enough memory.

there was not enough memory.

there was already a copy of Volume Menu running.  The first one to load will be working fine though. (Remove the duplicate copy to solve this problem)

a vital resource was missing.  Try installing a new copy.

your system is outdated.  Volume Menu requires system 6.0.4 or later.