Modern Site Script Support

Before activating or using Mega Drop Down on Modern sites, a feature called Script Support needs to be enabled on the site collection.  This is a requirement from Microsoft.  The feature only needs to be enabled once and only applies to SharePoint Modern sites. 

SharePoint 2019

1. Connect to your SharePoint server as a Farm or Site Collection Administrator
2. Open SharePoint 2019 Management Shell as administrator
3. Execute the following PowerShell command:
   
   

   (Get-SPSite -Identity "http://sharepoint.example.com/sites/modern").DenyPermissionsMask = [Microsoft.SharePoint.SPBasePermissions]::EmptyMask

   Replace http://sharepoint.example.com/sites/modern with the URL to your site collection

Office 365

Please use the steps listed in the following Microsoft article.