Sounds like either an enemy fleet is in the in the same province, you set the org threshold to high, or you set the timespan in which amphibious assault to short. When you set the order to invade, set the org to around 10 or 20 and set the timeframe to as far out as you wish. I usually order it to amphib attack for 3 months. Obviously you'll win or be defeated before then. But my guess is there's an enemy ship or fleet in the area. If so, sweep the area with your own fleet. Also, don't attach any escorts to your invading fleet. Let the transport work on their own.