Category organization and update errors

[metazai]

Magento 2.1.7, Mega Menu extension installed.  Whenever I try to make any change to any sub category (not the main category) I either get a 504 error or "Default Product Listing Sort by does not exist in Available Product Listing Sort By."  Have reindexed, recompiled, and flushed.  Not sure where the problem is or how to troubleshoot it as i am new to Magento 2.  Any help would be appreciated.

[Robert_Rand]

Which developers wrote the extension? 

Have you opened a support ticket with them yet?

[metazai]

I have, in fact,but they tell me to reindex (I have) or make changes to the category (which I can't). Mega Menu (by Zemez) is a popular extension.  I've searched the web for some sort of manual removal of the subcategory menus in question, but no dice.  Does anyone know if I'm looking for MySQL tables or a flat file?