If this is the first time you build a system and your motherboard is complaining about your RAM, you may have seated it in the wrong slot. Consult the motherboard manufacturer's manual to see how to mount your RAM properly. Main boards are very particular about this and will refuse to work unless the right slots are used. Usually, the lowest-numbered slot should be occupied with one stick. The second stick is mounted two slots down, then the third is mounted in the slot between the second, and so on.