Yes, I am talking about the i/o panel. It is a press fit into the case, and is often difficult to get in completely. You need to push hard, and work your way all around the edges so that it sits fully in.
Then, when you install the motherboard, you need to push the back panel of the motherboard into the panel, sometimes very firmly so that the motherboard screws can go in. Be careful that the grounding tabs on the panel are not interfering with the motherboard. I try to locate the first screws in the holes nearest to the back panel first.
If you have an extra hand, it helps; one to push and hold the motherboard in place, and another to put the screws in the holes. I find a #2 magnetic tip phillips screwdriver to be very useful for this.
Once you get two screws in, the others should fit. Do not tighten up the screws until you get them all in place.