What case are you using?
I have never heard of a case that did not have the tabs cut out to have add in PCI/e cards (which is what you are doing).
Take a look at the case right next to the motherboard - it could be that one of the slots where the tabs on the GPU are supposed to go got bent and closed enough that the tab won't fit.
If that is the case I would pull the motherboard out of the case (so not to get damaged) and using a flat screwdriver or similar pry the tab open until you can put the card in place and screw it in as normal. Once you can screw the card into place, remove it, replace the motherboard, and re-install the GPU.