In MacTracker it says that OS 7.5.5 was the original OS for the 6500 and it needed a System Enabler, so it may be that 7.6 also needs an Enabler (though I always thought System Enablers went by the wayside with 7.5, it was only 7.0/7.1 that needed them). Mac OS 7.x was never really meant for PowerMacs though, it is mainly 68k code and was hacked on to the PPC, so it could well be that it needs weird enablers etc - I wouldn't bother with it myself as it's not what I see as a PPC OS, though it should be possible if you really want to try it. The disc isn't a machine-specific disc by any chance is it?