If marking columns within particular session are marked as view-only you will not be able to enter grades into the marking columns in Enter Grades by Course or Enter Grades by Student sections. 

1. Navigate to Configuration
2. Select Academic Year
3. Select and open the session
4. In the "Set Marking Columns as View-Only in Grades"  box, Scroll to the marking columns you would like to enter grades for and un-mark boxes. 
5. Next, Navigate to Grades> Enter Grades by Course.
6. Search and open the course.
7. Enter grades into marking columns
8. Save and Close the Record