There is always a reason specified. I suspect technical issues!
The hearth of the flower is not crisp sharp and there are artefacts in the blossom.
If you are new to stock, you should consider these resources: https://helpx.adobe.com/stock/contributor/tutorials.html
Please read the contributor user manual for more information on Adobe stock contributions: https://helpx.adobe.com/stock/contributor/user-guide.html
See here for rejection reasons: https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
and especially quality and technical issues: https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html
The framing/composition of the flower is also bad. I think you should show the whole flower and not cut off the petals.
Along with the other reading materials, also read this!
You should also follow the links in the guide.