You'll need a course with at least one activity and at least one student. Make sure the site setting "unlimitedgrades" is off. Make sure the site setting "grade_report_enableajax" is on. Purge caches using the purge cache link at the bottom of the page. Go to the grader report and click on a student grade. A text box will appear. Enter a grade that is above the activities maximum grade. Press enter on the keyboard. An error dialog with a non-transparent background should appear. Enter a valid grade. The grade should be accepted.