This is an original photo, no filters or post-processing done. It was rejected with reason 'Artifacts Problem'.
I didn't really understand the explanation. Any comments / suggestions please?
Some people think "artifact" means "something artificial added". No, it is a normal effect that can be seen in many photos, and also can result from JPEG saving. So, since you did not change the image, look to the other causes listed in the message. The checker will have viewed your image at 100% magnification and looked at all of it, not just the main subject.
So, I guess this was taken using JPEG, which is the type of file format that cameras use to save images. Was this taken with a smartphone? When pictures are saved in JPEG format, esp with smartphone cameras files are compressed to save space. This means that you can get JPEG compression artifacts - the pixels appear 'blocky'. This is probably what you have here. You should view your picture at least 100% to look for these types of problems.
Contrary to what you seam to think, you are allowed to modify your pictures. I do this all the time to make the picture better and to eliminate defects that are on the picture. Sometimes that are quite sofisticated edits.
Pictures for stock need to be technically perfect and need to look natural. The buyer needs to have all the options to use the picture as he wishes. Artefacts will harm his options.