Secure the motherboard using the appropriate standoffs.
Here is the information you need to find the correct manual, as "21 B6 E1 E2" is likely a regulatory or batch number rather than the actual model name. intel desktop board 21 b6 e1 e2 manual better
socket boards from the 2nd and 3rd Generation Intel Core era (Sandy Bridge/Ivy Bridge). Secure the motherboard using the appropriate standoffs