It may simply be, as stated above, that your PowerMac isn't compatible with the drive. When that Mac was made SSDs simply weren't available on the general market, so it wouldn't be a surprise if there is a compatibility problem.
Look at the model info on the SSD manufacturer's web site to see if there is a compatibility switch or jumper setting that may help. Reading more carefully I see that's already been suggested, though repeating it might help a tiny bit.