I believe there's a bug in the Web AppBuilder developer edition.
When a feature service layer has labels applied to it (create labels with AGOL, not labels are part of the map service) within the referenced AGOL webmap, the Edit Widget in WAB will not recognize any feature services. If labels are not applied to any feature services within the AGOL web map, the Edit Widget in WAB will work.
I also believe this is specific to the developer edition of WAB. If I use the WAB from AGOL on the same web map, and feature services have labels created, the AGOL WAB will work.