How can I improve the quality of my photo?

These look like phone camera snapshots taken indoors without a flash and uploaded to Stock without any corrections. 

That's not how we do things here.  Stock customers expect commercial quality assets for use in professional projects. They won't pay for images that aren't even worthy of posting on social media.

Your images are poorly lit & lack details & proper composition. That's why Stock rejected them for Quality issues.

• https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
• https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html
• https://helpx.adobe.com/stock/contributor/user-guide.html/stock/contributor/help/photography-illustrations.ug.html

Photography is a lot more than point & click. If you're serious about learning photography, enroll in some online courses & practice, practice, practice.

Good luck.