Repairing machine, tried to install optical drive, can't insert again

Hi,

I just want to start off saying that i am NOT a Mac specialist at any point, i mostly work with Dell/Lenovo enterprise and custom bulit Windows PCs, and NOT Macs. I offered to fix a friend of a friend's iMac 17", as they couldn't find someone to fix it, and it's one of their only few computers.

I removed the PSU, and i had to completely dissasemble the machine, and put it back together. At this point, i am trying to re-insert the optical drive, but the riser for the optical drive won't line up/connect with the optical drive port on the motherboard. Can anyone please help me?