I , an Instructor, assign CK12 adaptive practice through canvas and I am happy with it for the most part. (I would like to select multiple chapter sections at once, but that is by-the-by. )

I set up the assignment so that CK12 is opened in a new tab.

Some (~10/50) of my students encounter an error message when hitting the CK12 "Turn-in" button after they have finished the adaptive practice. "Grading not enabled" was part of the error message. the students are doing the work in the classroom on Chromebooks

As a work around to the problem, I have students close the CK12 tab, go back to the assignment in canvas, open the CK12 tab again and then hit the "Turn in" button. This works.

So it seems that the linkage between the two platforms becomes unstable. It is possible that it is a network issue but I'm wondering if any one else has encountered the problem and found a more tangible solution.