Noise reduction, upload to shutterstock declined

The image is a bit "jpeggy", there is artefacting in the flat colour areas & at high contrast edges which a machine AI system might reject.

Example of jpg artefacting at contrast edges

The image size is also a bit small for a modern stock library; it's about 3mp compared to the 20mp or more that they might be expecting to see these days.

I picked one area of the photo to concentrate on - plain sky with one small cloud - where it's easy to see what's going on.
These are screenshots from Adobe CameraRAW, showing the controls on the right.

^{Click any of these to see full size, too small to see detail in here. Had I noticed it earlier, I'd also have healed out the obvious dot in the middle ;)}

Original pic as uploaded

With added Sharpening - this really picks out the 'jpeggy' edges, something an AI might do to see how much compression was used (this would also pick out other issues such as film grain on analogue-sourced images or colour aberration in any image).

Removing the sharpening & adding some smoothing (noise reduction) instead has the unfortunate side-effect of also removing detail from the cloud

You could perhaps try to balance the two together to retain some detail whilst flattening the broad coloured areas (this isn't perfect, just a hint in the right direction)
You could also get in there with a more detailed brush approach to smooth & sharpen specific parts.

There's also some colour aberration - a lens defect - which is giving green & purple fringing to the image edges.
Again, CameraRAW can have a go at trying to reduce this effect…

If your original photo was shot RAW rather than jpg & also if it was shot at a higher resolution than 3mp, then go back to that & see what it looks like. When exporting to jpg, save at full size & 100% quality (or save as png) if the site will accept that file size (they ought to do.)