no secondary menu on tablet and phone

Dear,

I'm trying to 'lose' the secondary menu when you visit the site on a tablt or a phone.
So I foun dthis code from earlier issues you guys handled :-):

@media (max-width: 768px) { .secondary-navigation { display: none; } }

But it does not seem to work? What am I doing wrong?

kind regards,
Nele