Hi, I need help fixing my Dell optiplex problems

I have bought a Dell Optiplex 5040 MT, it has 16gb ram, i7-6700, and 1tb hdd.

I bought a rx6600 at a good deal, and I also bought a lien liu 750w power supply. I used the often accepted solution of using a 24 to 8 pin adapter, however my motherboard is like killing itself and its showing a double flashing amber light whenever I use the new power supply, the old one works fine, I am just trying to get this working cuz this is the only way my parents will let me get a PC.

PLEASE HELP FAST