When running the Create Notification task from Portal Sharing REST API, the following error message is returned, "Error 500: You do not have permissions to access this resource or perform this operation".
The Create Notification operation in the Portal Sharing REST API is not supported in ArcGIS Enterprise or ArcGIS Online. As such, the Create Notification operation has been removed from the Supported Operations list in Portal Self starting in ArcGIS Enterprise version 10.9.1.