It refers to needing a microcode update. Newer CPUs needs one to be fully supported, they are built into bios updates. But usually if the CPU is supported enough, so that the computer boots, it should be possibly to bybass the error. The computer would pretty much function fine. But if that model doesn't allow bypass, you need to update the bios.
I think the computer you have is named IBM ThinkCentre M55, some models may have different motherboards under same model name, in this case that would be type 8812. I don't know where you pulled the "A15" from.
Anyway based on that, hopefully the page below has the BIOS update for your machine. They will always check the machine before they do the update so if it complains about being for...