I had created an assignment and used Submission Type: External Tool.
I was presented with
External Tool Options
I had entered the URL, completed the assignment unit setup, saved, and then proceeded to click on the assignment post. I had received this error: Couldn't find valid settings for this link.
However, the assignment would not allow me to get back in to Edit. Everytime I hit edit, I got that error message. Then I had to go on a lengthy, teacher determination, patient, click-here-click-there-click-everywhere, speed reading, hold back the screams journey to resolve this ridiculous problem. Of course thru any fix-a-problem journey, you discover & learn new things to that'll test your critical thinking skills. Nice but time is loss on the work that really needs to get done.
In summary, I went into settings,
found the assignment in error,
clicked edit,
GOT IN, and
clicked: No Submission.
DONE!!!
But a lot of time loss!!!!
Why didn't Canvas allow me to edit from the Module/Assignment page rather than the Settings page???????????
WHY??? 
