The metal tabs are supposed to be on either side (and top) of the ports on your back panel.
Those tabs are there to keep the ports stable in place when you plug things in.
You will need to remove the mobo and re-place it in the case.
Before you do this snap the back panel cover onto the case and push any tabs back.
Slide the mobo into place making sure that all the ports are clear of their tabs.
Hold it in place and put in the screws.