You probably have blank criteria in the assessment form. I had this. I had copied a previous workshop and then edited the assessment form. I deleted the content of the final criterion and the possible scores in the visual editor. However.... It was saved as having a return in the html giving a single blank cell at the bottom of the assessment form. It looks like you have seven blank criteria. Go in and turn on the html editor for each of those criteria and delete the break.

Students haven't completed the assessment so it doesn't save.

But, they can't complete because there is nothing to select.

Basically the assessment form editor should not allow null criteria or criteria without possible responses. It shouldn't let you save that assessment form and should red box it and asterisk it like everything else.

Also, if this does sneak through instead of just not saving, Moodle should give a "you must select" message. That would have helped me to identify this problem quicker.

Hope that helps.