If the User object is not visible in the "Map parent records from Source to Destination org" section, it likely means that the mapping for the User object has not been configured in the custom settings. To fix this:

  1. Go to Setup → search for Custom Settings → click "Manage" next to Source to Destination Mapping.
  2. Click the New button.
  3. Fill in the following fields:
    • Name: Enter any name (e.g., User Mapping)
    • Destination Object External ID Field: Enter the API name of the external ID field in the User object (e.g., User_External_Id__c)
    • Destination Object Name Field: (Optional) Enter the API name of a field you want to display in the result table, or leave it blank.
    • Destination Object: Type User
    • Source Object External ID Field: (Optional) Enter the external ID field API name from the source org, or leave it blank if you want to use record IDs.
    • Source Object: Type User
  4. Click Save.

Now, when you go back to the "Map parent records from Source to Destination org" section, the User object will be available for mapping.