Often the reason why an x-rays can not be attached to a claim is because of the modality is saved under a different modality not match with DentalXChange.
To fix this find the x-ray you want to attach in Dentiray Web, notice that this pano was saved as an XC but for DentalXChange it needs to be PX for pano. You will need to select the pano and press the Edit button on the lower right side.
From here you can change the modality from the dropdown to PX, then press Submit to save.
Now when you head back to Denticon to attach the x-ray to the claim it will now appear.