Sounds like you are in a restricted privilege set or have a custom menu installed. Are you the original developer of this file?
If you know you have a full access account, then it's likely the custom menu. In FileMaker Pro Advanced, you can go to the Tools > Custom Menus > [Standard FileMaker Menus]
which should disable any custom menu put in place.
If you are not the original developer and do not have a full access account, you will need to contact the original developer to get your privileges unlocked.
Are you logged in as a user with Full Access privileges?
Is the window a Card Window or Modal Window? Is there a script running?
Try going to a layout you can modify, go in layout mode. Using the dropdown layout selector on the toolbar in the upper left, see if you can navigate to that layout.
Mike has the idea.
There is an option. You are trying to edit the layout with the layout active. This is probably blocked by the layout being assigned a custom menu in the editor. There is a popup menu for that.
You can bypass that by editing another layout, opening the layout dialog and scanning for the layout in question and temporary reassigning the standard menus remembering to change this back when done.
If the custom menu assignment is done in a layout trigger, you can temporarily wrap that step in the script with
Remembering to remove your if/endif when done.