CAnnot add page to menu

I am running WordPress 4.6.1 running Twenty Ten theme. I am unable to add more pages to my menu. I Have created the page, published it, and it shows up on the menu page. I select it and add it to the existing menu, hit save and it goes to “page not found”. I even tried doing it with a page that is already in the menu and it give me the same error message. I have turned off all the plug-ins and updated my theme and plugins and it still doesn’t work.
I am a pretty inexperienced uses so I don’t have a clue about debugging and no I have not changed any of the code, as far as I know. as suggested in other forum posts.
Any suggestions?