Hey, you are a terrible photographer.
I can't tell anything from the photo, but there are only a couple of things that can be a problem. On the edge of the motherboard there are headers for the USB and audio and perhaps fans. If the GPU needs to seat above them, the cable may be in the way. The output side of the board needs to have the metal tabs slip between the motherboard and the chassis, often into little slots in the chassis just below the level of the motherboard. Sometimes you need to bend the GPU mounting bracket slightly to ensure the tabs line up. Of course, make sure that all three of the blanks and screws are removed from the chassis first.
Finally, there may be brick-a-brack or heat sinks or RAM interfering with the area the GPU needs.
Also, make sure to insert the board level, trying to get the brackets and the PCIex edge connector to go in at the same time. If you are locked under the clip with the bracket end not in the PCIex slot, then the key in the slot may be blocking the board from rotating down into place.
Most often, the problem is with the bracket side not aligning properly. Unclip the PCIex clip and try again, aligning the bracket carefully. Sometimes I have to bend the bracket slightly to get it in position when inserting PCIex boards. It's slightly possible that the motherboard is not well aligned or you used the wrong standoff positions when you mounted the motherboard, but if the IO plate is well aligned then that is unlikely. You can slightly loosen the motherboard screws to give some wiggle room if needed.