I, too, was having issues configuring WeBWorK with my institution's Canvas LMS. In particular, I kept getting the "Couldn't find valid settings for this link" when trying to set up grading in "Homework" mode. I wanted to share what worked for me here.
In Canvas' app settings:
-- "Launch URL" must be set to the course. For me, it had to read "..../webwork2
-- "Domain" does not need https:// or http://
My mistake (I believe) was leaving Domain blank. I hope this helps someone.