Remove the hold down screws on the i/o panel.
It appears that you have done this.
The more tricky part is the latch holding in the opposite end of a graphics card.
How they function differs.
Worse, it is under the card where you can't see it easily.
Some work by pushing down on the latch.
Some may work by lifting.
Others work by moving the latch to the side.
If you are certain it is unlatched, you need to lift the card out evenly.