Because the 845K is not on the CPU support list, the only models it supports is the 840, 850, 860, 870, and 880, the 845k is AD845XACKASBX
FM2+ Athlon X4 AD840XYBI44JA 65W Kaveri 3100MHz 2MBx2 A1 P2.40 FM2+* Athlon X4 AD850XYBI44JC 65W Godavari 3200MHz 2MBx2 A1 P2.70 FM2+*◊ Athlon X4 AD860KXBI44JA 95W Kaveri 3700MHz 2MBx2 A1 P2.40 FM2+*◊ Athlon X4 AD870KXBI44JC 95W Godavari 3900MHz 2MBx2 A1 P2.80 FM2+*◊ Athlon X4 AD880KXBI44JC 95W Godavari 4000MHz 2MBx2 A1 P2.80
You can find detail information on your motherboard from ASRock > FM2A55M-HD+ , and your processor, http://www.cpu-world.com/CPUs/Bulldozer/AMD-Athlon%20X4%20845.html . There is nothing preventing this motherboard support your processor, but you have to install an external graphics card to make the system work. Even though it is not listed in the support list, but I believe this combination would possibly work for you.