Why my photo was rejected for technical issues ?

Hello,

Well, I think the overall shot is a bit too contrasty - the shadows are blacked out. As you took this in bright daylight - summertime?  you are going to get harsh shadows, which is what you have here. There are also signs of noise in the sky:

Another thing you need to ask yourself is, how could this image be used commercially. What are you trying to say with this photo?

Have a read of this. It's a brief guide on image quality:
https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html

and
https://helpx.adobe.com/content/dam/help/en/stock/contributor/Adobe_Stock_Contributor_Guide.pdf
https://helpx.adobe.com/stock/contributor/help/photography-illustrations.html

to get you started.