If you can get to the page by typing in the URL then the developer that built the site has hidden the menu I do this all the time to prevent clients that don’t know what they’re doing from massing about with the field groups I’ve set up.
Like I said, there are several ways that hiding the menu can be accomplished. One is with code in your theme’s functions.php file.
Another way is to install an advanced permissions plugin. You may be listed as an admin but you many not have all the permissions of an admin and you wouldn’t be able to see the settings for the permissions plugin to find this out.
If you really need access to it you should probably contact the developer that built the site and ask them how they’ve hidden it.
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