I thought I would share an experience another teacher and I had to problem solve today in case it may help out others that find themselves in a similar experience.
A teacher was creating a Canvas assignment that was not a cloud assignment. The teacher was making it a regular, but non-graded assignment. I was called in when her assignment was unable to be saved or published. She did not want to lose everything that she had created, but it just would not save. After a little bit of problem solving we found the issue. (Intrigued yet?)
Turns out there was a conflict between submission type and another setting in the assignment, but it was hiding from us. The teacher had been creating an Office 365 Cloud Assignment earlier in the day using submission by external tool. When the current assignment was created, it defaulted to external tool for submission type. However, the teacher had selected "Not graded" in the Display Grade field. This action hid the options for submission type. So, even though there would be no submission, the assignment was looking for a connection to an external tool that was never made, not allowing the assignment to be saved. When "Not Graded" was changed to "Points" (or probably any of the other graded options) the submission type appeared and could be changed to something other than "External Tool". Then, the assignment could be set to Not Graded again and saved.
Here is a screen shot of what I saw initially. Notice that "Not Graded" is selected and the menu for submission type is not visible.
After changing "Display Grade as" to "Points", it was possible to see that the submission type was set to external tool.
Really a quick little fix...once you figure out what the problem is. Hope this helps in case someone else runs into a similar circumstance.