Okay please share admin cardentails in private reply for better solution and also
mention a blue print which elements you want to insert in this mega menu,
Thanks,
OK first please create h blank page like title with blank okay after that go to
appearance>>menu and select a menu after that add this blank page to selected menu and
remember always it is visible for only and only root level menu item,
Refer: https://prnt.sc/rwjr4d
If any issue pleaase provide me admin cardentails i will create a demo on your website,
thanks,