It may be that your 9.1 CD isn't made for whatever Mac you're trying to install it on, or you have to update some Firmware first.
If you already have OS 9.0 installed on the Mac, I'd suggest using the upgrade path detailed
HERE, and download the installers listed there as well.
Please note, you HAVE to update 9.0.x -> 9.1 -> 9.2 -> 9.2.1. You cannot skip directly from 9.0.x to 9.2.1 using these updaters. Also, you should boot without extensions (hold down shift at boot) before installing these updates.