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.