The problem is fixed. For those interested, I followed the advice at:

https://www.zen-cart.com/tutorials/i...hp?article=142

The author of the above article says:

"Leave the space between the quotes for the template screenshot field empty for now, since you don’t have one yet."

I did leave it empty and that caused my problem. I fixed it by putting an image file in that space. It probably would be a good idea to edit that part of the tutorial.