Cannot set featured image

Hello there,

After applying the theme on my page, I realized some of featured image in my posts were not shown properly. And when I opened edit post section, my featured image in the post became error. I tried to upload other picture to replace the featured image but it seemed not work at all. On the other hand, when I chose to use the old pics in my media, the featured pic was shown normally.

I wonder if there is a flaw in the coding/ whether I had done something wrong during the setting process?

And also, the link of my wordpress is: nikkinguyen.vn. Please check it out and give me some feedback.

Thank you!