I thought I would put this out there in case anyone else was having an issue with the Web App Builder Print Widget. I had a web app builder project that was giving me errors when I tried to print using the print widget. Through a pretty cumbersome process of elimination, I discovered that the print widget was failing due to one of my feature services in the web map I was referencing, was using a complex label expression (using the new Arcade interface to build it). When I took the expression off and used a simple expression, the print widget worked.