See if this helps you. Be sure to read the short description carefully to understand the problem that this plugin tries to solve, and why it’s only a temporary solution.
Thank you for the suggestion, I tried it but it did not work.
It is a difficult one as even disabling all plugins and going to WP 2020 theme the issue is still present, I cannot add new menu items.
The cause of my problem with not being able to add new Nav Menu items via Appearance -> Menus was a corrupt WordPress database table (x2).
The corrupt tables were the wp_posts and wp_posts_meta and were missing the unique identifier and also no AUTO INCREMENT set for the ID field.
Now to fix this took some more work as with the Unique identifier not being set, the cPanel PHPMyAdmin is not able to edit the tables.
I could not add the unique identifier as the tables had duplicate entries for the default WordPress field for unique identifiers for each table in question.
So I needed to export the tables in question, remove the duplicate fields, rename the original tables so I could import the modified tables.
Once I imported the modified tables (with the duplicate ID entries removed), I was then able to add the Unique Identifier to the required field and add the AUTO INCREMENT attribute for the ID.
Problem solved… easy hey….. not.