The Centris SIMM you have is the right kind for your Power Macintosh 6100, but you need a matching pair to do the upgrade. The Centris has a 32-bit wide memory system. Since the SIMM is 32 bits wide a single SIMM is all you need to add memory. The Power Macintosh 6100 has a 64-bit wide memory system. This requires two matching SIMMs to perform an upgrade.
If you only install one SIMM, the computer still starts up, but it will not use that memory.
The same two SIMM installation requirement holds true for the Power Macintosh 7100 and the Power Macintosh 8100. Later Power Macintosh computers could be upgraded with a single SIMM or DIMM.