If you cannot grade a subject (example in the picture below), it means that that subject / discipline does not have the evaluation type set.
In order to be able to grade on a subject you must first setup the evaluation type for subject. To do this, go to "Activity Schedule" and in the list of activities on the left, click on the gray / white button next to the activity you want to change (if you do not see the list of activities to the left of the schedule, click on the gray button "All activities" located at the top left of the page).
In the window that opened, you can see how the activity is configured. In the example below, for the subject "Music", next to "Evaluation Type" is set to "No evaluation". To change the settings, click on the green "Modify" button.
In the next window, scroll down until you see "Evaluation Type", then click "No Evaluation" and select the desired rating type. After you have finished making the changes, click on the green button below called "Save".
The next step is to reschedule the activity if it is scheduled in the past (an activity scheduled in the past will keep the settings it had when scheduling). To do this you must first delete it. On the "Activities Calendar" page, click on the activity in the calendar to be rescheduled.
Warning! If the scheduled activity has attendance or has other types of evaluations granted they will be deleted from the system. You can keep the history very easily (click on the present / absentee under each student to reflect the attendance before the deletion) but the previous evaluations will be deleted.
Now click on "Edit activity details", then click on the red "Delete" button and confirm the deletion.
Now all that remains to be done is to reschedule the activity. After programming it, click on it and set the attendance and grades.