Can someone please explain why my picture were rejected?

Since you’re new to Stock, read through the requirements in https://helpx.adobe.com/stock/contributor.html. 

Start with the 4-part photography section.

Customers expect commercial quality content that can be used in professional projects— print, digital & merchandise.

Before you submit, compare your best work with similar Stock inventory. Yours should be as good or better than what Stock is selling and offer customers something unique from the rest.

https://stock.adobe.com/search?k=wallnuts  1.3 million results

https://stock.adobe.com/search?k=wallnuts+bowl  222K

https://stock.adobe.com/search?k=landscape+mountain  32 million results

Hope that helps.

The walnut image could be improved. I love minimalist composition, but maybe this is just TOO simple. Maybe some cracked walnuts and a nutcracker on the sides of the bowl would give the image more interest.