003988: Access to this resource is forbidden, regardless of authorization.
Description
The client code in ArcGIS Pro failed while attempting to make a connection or query to the server.
Solution
Check your network connectivity, server capabilities, and server logs. This will help determine any potential causes for the failed request.
If you encounter this error when using the Append tool with the Optimize performance for feature services parameter checked, the target feature service may not have the Uploads capability enabled.