Unable to add a field in the attribute table of a feature collection

Description

Users are unable to add a new field to the attribute table of a feature collection in ArcGIS Online.

Cause

This is by design. New fields cannot be added to a feature collection because the changes done to the feature collection are only temporarily reflected in the web map, and not applied to the original data.

Solution or Workaround

1. Log in to ArcGIS Online, and open the My Content page.
2. Click the drop-down arrow next to the feature collection layer (listed as Feature Layer under the Type section), and click View item details.
3. Publish the layer. A hosted feature service is now created. For more information, refer to ArcGIS Online Help: Publish a feature collection.
4. Open the hosted feature service in the map viewer.
5. Open the attribute table by clicking the Show Table button from the Contents window.
6. Click Table Options > Add Field. For more information on adding a field to a feature service, refer to ArcGIS Online Help: Add a field.

Note:
Only the owner of a hosted feature layer or an administrator of the organization is able to make changes to the attribute schema.