Why my photo got rejected?

The white balance should definitely be warmer. The yellow trends to green. I also see posterization, but that may also be the JPEG compression here.

I think it's the blown out lights and the lack of detail in the depths. You should check the histogram.

Here you can find out more about the technical requirements: https://helpx.adobe.com/stock/contributor/user-guide.html/stock/contributor/help/quality-and-technical-issues.ug.html