I've attached a photo of a bay sunrise which was rejected due to artifact problems. Due to the low light? Post-production wasn't very much, so any helpful comments would be appreciated.
Your image here is too low resolution to point out the artifacts. However, if you view this at 100% or so, you probably will begin to notice the pixels becoming 'blocky'. This would be JPEG compression. Was this taken on s smartphone? As this was taken in low light, noise may also be an issue. Again, can't tell at this resolution.
The color of the sky is not smooth. It seem to be grainy. As highlighted by Ricky, zoom the image up to 100% or more and inspect it for blotches, artificial colors, and grains. It is bet to set your camera to the correct exposure before taking you picture. In this way you have less grains and artifacts on your image.