When I use the Print Widget in a Web AppBuilder Application the basemap does not load in all cases. If I zoom out the basemap does load. A message is substituted "Map data not yet available".
This message in the print output means that you've zoomed into a scale where the basemap is pixelated and not at a fine enough resolution to print from the Print widget. I believe if you zoom out to a "lower resolution" then the map should print out as expected.
Hope this helps,
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