I did eventually get 10.8.1 web adaptor to work with ArcGIS Server and portal on Ubuntu 18.04 but it was a bit complicated. I re-configured tomcat to use ports 80 and 443 instead of 8080 and 8443 (which are the default tomcat ports) before I could get it to work, (it probably would have worked with the default ports but I didn't want to use them), and also use the a specific version of tomcat. From memory I don't think the doco gave good examples of using the default tomcat ports to register the webadaptor with server and portal.
I also found that I had to manually install a specific version of, tomcat 9.019, to also get it to work. The tomcat install from the ubuntu repository (I think it was 9.0.21 at the time) I couldn't get it to work. Esri support also confirmed this. I have attached the rough process that I used as a txt file as the formatting in my doco didn't paste well into this editor.
You will have to adjust it for your usernames, paths, server names etc. I didn't end up using web adaptor on linux because I think one of the limitations was that it couldn't do domain authenticated users, which I wanted. So I ended up going with web adaptor on IIS linking to our ArcGIS Server & Portal on the linux server.
Hope it helps and saves you some time getting it going.
Regards
Steve