I am afraid this might be due to a known issue MDL-49779 with the Number of errors editing form.
Please go to your workshop and edit the assessment form again.
At the bottom part of the page, there is a separate form in the Grade mapping table section.
That form is used to define a mapping from the number of errors ("no" answers in your case) to a grade. You have to set the mapping manually and safe this mapping form.
Once you do, you will probably need to have the workshop in the assessment evaluation phase, and press the Re-calculate grades.
More details can be found e.g. at https://moodle.org/mod/forum/discuss.php?d=198442