You have many pcie x16 slots and it can take much power to run them all.
That is why your motherboard has a place for 4 and 8 pin atx power.
You likely do not need all the power but plug them both in anyway.
Your psu has two 8 pin eps 12v connectors.
Use them. One of them will be plugged only half in to the 4 pin motherboard connector.
Be careful that the other end of the modular cable is plugged into the eps12v port on the motherboard.