I am trying to find out why this image didn't pass the quality control. Any input would be appreciated

Hello,

Compression artefacts is one reason for quality rejection. This often happens with smartphone photos.

The pixels get smudged together - lossy compression.

The built-in processing in most phones, particularly iPhones, is quite aggressive. I frequently have to dial back the saturation on my iPhone images.

I like the image but phone cameras can’t compete with high-megapixel cameras. Your equipment will be your biggest challenge with Stock.  Start saving for a good entry level camera & lens.  

No, that’s haloing, which often happens when dark subjects meet lighter areas. It can be removed in Photoshop. There are a number of tutorials on YouTube. Search how to remove haloing in Photoshop.

Seems a bit over-saturated, and there is some banding in the sky.

Some haloing along the hills in the background may be one reason.