I find myself in the same situation:
1.- Launch Doctopus in a new spreadsheet in My Drive (personal, not team Drive)
2.- Select Classroom mode
3.- Ingest assignment
Then I try to attach a rubric form My drive, by clicking on Attach Goobric and selecting in Attach rubric
I get the same error than Anna & Peter. (Oops...!)
However, I found a kind of half-fix by refreshing spreadsheet and re-opening Doctopus.
Magically, link to rubric appears on top right corner. So, the rubric is loaded.
Then, by clicking "Display latest scores", I can see "rubricScores" tab.
After that, I can use Doctopus "normally".
I hope this helps you, to use Doctopus and to find a fix for that issue.
Thanks,
Antonio