iMac 2019 27" CPU change

Hello, I tried changing CPU ( i9 9900) and ssd, but after assembling everything, it does not start. I tried searching online what the cause could be, and looking at the diagnostic led i could not find a solution. As you can see from the image there is one led on, and if I understand correctly it is the #2. I tried disassembling again to see if there are some bent pins, but they are all fine. What could be the problem?

Thanks