background image problem

can anyone hel me understand what’s the problem here? https://test3.hscreativedev.co.uk/blog/

if you click and inspect on ‘As seen on’ you would see that I am trying to insert an image background, but no success…

Any thought on what am I doing wrong?

here’s my code:

#menu-item-15 {
background-image: url (images/test.jpg) no-repeat fixed center;
}