Are you using Web AppBuilder for ArcGIS Developer Edition and with Portal for ArcGIS 10.5.1?
FYI: we currently have an open bug with the Add Data widget in Web AppBuilder Developer Edition and using it with Portal for ArcGIS 10.5.1 (BUG #000105062) when attempting to add a shapefile. Although we've seen different error messages returned.
Can you please contact Esri Tech Support, mention this bug, and provide more detailed repro info so we can investigate further?
Hope this helps,
I am having this issue as well using Portal 10.8.1 but with using a CSV file.
As a workaround I figured out I can use the Geo Lookup widget. I didn't want to enrich any of the layers because there are too many polygons, so I removed the layers from the Geo Lookup widget config file (using Web AppBuilder Dev Edition).
}, "enrichLayers": [ ]