Hi and welcome to WordPress StackExchange. As a rule of thumb: the more information you provide about your problem, the better people can help you.
I assume that this error occurs on your development server?
In case this warning appears on your live system, I’d recommend to disable php error reporting, not to surpress the warning message, but rather to hide security issues for outsiders. Just a thought of mine.
The error message says, that the “WP menu walker class” exceeds the level of nesting.
The WPMenu Navwalker Class is the php-class, which is responsible for building the HTML menu structure of your theme. It basically says, that there is an error when building the menu of your theme.
Since you are using a commercial theme (Mentis), you have to contact the theme developer in order to help you/fix this issue.
If somebody fixes it here with a solution for you, then within the next update that fix will be overwritten (unless you build the fix as a plugin or child theme).
Another solution might be to remove the 4th level of nesting in your backend-menu. That way the Theme might be able to render 2 or 3 levels of nesting.
Like this:
Menu level 0
|> level 1
|> level 2
|> level 3
|> level 4 << remove item in dashboard >> design >> menu
Try removing level 4, if that doesn’t fix it, try removing level 3 etc.