The Web AppBuilder for ArcGIS fails to print when the Basemap Gallery widget is used to toggle between Web Map Tile Service (WMTS) basemaps using the non-Web Mercator projection. This bug is 000119846.
The work around for this bug within AGOL when using the Web AppBuilder requires using Stream Raster as a layer in the Layers widget. This can be accomplished by creating a Map and adding every one of your Stream Raster layers to it.
You can then add that Map to your Web AppBuilder. Once it's in the AppBuilder, the user can toggle Stream Raster layers using the Layer widget instead of the Basemap Gallery and have printing still work.
An alternative to this suggestion is to request a Web Mercator projection during ingestion into Stream Raster.