Background
When using the wp_nav_menu function, either a specific menu (by ID) or a theme location (associated with a menu) can be passed. However, if both are passed, the menu ID takes precedence.
The Problem
If a theme’s wp_nav_menu() call uses the menu parameter, the critical theme_location parameter will be ignored – and therefore UberMenu cannot target the menu.
Some themes use both the theme_location AND menu parameter. Only the theme_location parameter should be used.
Identifying the issue
You’ll need to find the wp_nav_menu function call and check the parameters that are being passed to it. This function is called in the header.php under normal circumstances, but you may need to do a global search on your theme files to locate it. If the wp_nav_menu call is passed a menu parameter in addition to the theme_location parameter, you’ve identified the issue.
If the theme_location parameter is not present, you have a secondary issue as well.
The Solution
You can either remove the 'menu' parameter that is passed to the wp_nav_menu function, or use Manual Integration (UberMenu Easy Integration if you are using UberMenu 2).