Hi wplms,
It sounds like you have a JavaScript error on your admin pages. Please check:
https://www.megamenu.com/documentation/javascript-errors-console/
The fastest and easiest way to locate JavaScript errors is to disable all plugins and see if the error disappears. Then, in order to discover which plugin is causing the problem, enable plugins one by one until the problem returns. You will need to notify the plugin author of the error if you want to continue using it.
If the error message is still present even when all plugins are disabled then the error will most likely be originating from your theme. Try switching themes to see if the error disappears. If it does, you will need to contact your theme author for assistance.
Regards,
Tom
