How can I improve my photo quality

Compare your best work with similar Stock inventory to see what's expected. And if what you're selling is something that Stock actually needs.  Everyone submits beaches & sunsets.  But nobody submits soda cans and cardboard containers. So keep that in mind when choosing subjects to photograph. 

Introductory courses will help you understand camera fundamentals -- proper lighting, composition, focal distance, ISO/f-stop.  But good photographers learn more from their mistakes than they do from their successes. So don't be afraid to make mistakes. Experiment & practice, practice, practice.  

I suggest you take courses about photography, but also about post-processing.

One way to make your photography "stand out" is with intensive post processing. That is why Adobe offers it's photography subscription. Here is an example of how I developed your photo using Lightroom and Photoshop. Total time, approx 3 minutes.
 

IMG_E7733.JPG - DOF is too shallow and it's too tightly cropped. Landscape orientation might have been better than portrait.

IMG_E6998.JPG  - the horizon is not perfect,y level and there's some chroma noise in the sky 

the horizon is also leaning in this one