You'll need to enable **Transactional** in the global WFS settings. Be careful with the workspace-specific service settings because they don't usually do what people think they do. Those settings only come into effect when you use the virtual endpoints such as http://localhost:8080/geoserver/myworkspace/wfs, but they have no effect when you are using the regular http://localhost:8080/geoserver/wfs endpoint.

You should also make sure that transactional WFS is secured so that unauthenticated users can't modify your data.