If you are getting an error while saving your navigation menu, the most common cause is a "duplicate" menu item. Meaning somehow an item was somehow added twice on your navigation menu, sharing the same menu item ID, which generates an error when saving.
- On the Edit Navigation Menu page, search for a link that has a duplicate name in the "Target Page" column.
- Uncheck both of the items that have a duplicate.
- Click to Save Changes.
- The navigation menu should save without error. If you still get an error, then you may not have unchecked the correct items.
- Re-enable the menu item that you turned off.
- Click to Save Changes again.