Seeking technical advice for image rejected due to "Artifacts".

How do you know the reason was artifacts? I see several technical errors with this photo.

1. the horizon is not level.

2. birds that are blurred.

3. focus is too soft.

4. no details in the sky.

5. artifacts around the trees.

6. not cropped to Rule of Thirds.

Landscapes are also better in a panorama format. Such as 3:1 or 16:9.

I would go for this look with the above corrections made:

Some artifacts are created in camera and some are created by excessive processing. It doesn't look like you did much processing, if any, to this image, so I would assume that the artifacts are a side effect of your camera's processing. This article provides more insight into what causes artifacts and how to prevent them:
https://www.lifewire.com/avoid-artifacts-in-digital-photos-493765

You have chunks of blocky pixels which are probably due to a small sensor; these cannot be corrected by processing. You have chromatic aberration which can be corrected in Light Room or Photoshop. 

You have some lens spots and blurry birds in the sky which can be erased in Light Room or PS.

You also have a rather overexposed featureless sky which can be darkened a bit in post processing, and a leaning horizon which can also be corrected.