404 error

Hi everybody,
I need help. I have a big problem:
on the top left of the pages of a website that I created I have a 404 message that forces the end of the head and puts everything in the body included metatags.
You can see the error on this image

The website is http://www.cesimitalia.eu
The theme is showycase.
I apologize if there is a discussion about this but I searched everywhere but I don’t find nothing about this
I apologize for my English too: I’m not fluent.

Thanks in advance.