How to unlock menu editing

1. Try with another browser.

2. Check your browser console : Ctrl + Maj + I Or right click and
   inspect.

   If you find error : First try to disable each plugin one after the
   other. In order to find out which plugin can generate the errors.

   OR

   -Modify the wp-config.php file and change (or add this) :

define( ‘WP_DEBUG_LOG’, false );
define( ‘WP_DEBUG’, false );

by

define( ‘WP_DEBUG_LOG’, true);
define( ‘WP_DEBUG’, true);

-Go back to your menu page and click on the arrows.
-After that open your debug.log file in the
wp-content folder and check for errors and try to fix them.