Q: Sku fetch did not put the main image in the description section, why?
A: Be sure that you have not made a modification to the template. It already had the code for the main image in that box, but it's on the back end in the HTML code as the tag {{main_image}} only inserts the URL for the image... but it needs to be surrounded by the HTML to display an image which can only be done in the HTML code. You can reset your template. You should be able to do that by clicking the Reset to Basic button and if not, you can switch to a different template then back to this one.
It will display as a broken thumbnail of an image but that is because (while it is just a template and no item is loaded) there is no actual image to display. This is fine.
Once corrected, save your settings.