Did you ever get this working? I have a similar issue - trying to deploy a Reportlab script as a Web Tool. It runs in ArcPro in our cloned environment just fine (both locally and directly on server) and packages as a web tool for Portal ok. But when I try to run the web tool from Portal, it isn't recognizing the reportlab.pdfgen library. I did use Conda to make a clone and installed reportlab into that environment (and activated it) but still no luck. We are running ArcGIS Server 10.6.1 and ArcPro 2.6. Any help is appreciated! Thank you!