Interesting problem. Well, REV02 which appears to the module your using only has one line item and it's a calculated line item so it can't be edited anyway. Although, you shouldn't be prevented from toggling to make it editable - I can't seem to repeat that problem. If you really want to edit the data I would suggest using DATA02 but that's at SKU level.
Try rebuilding the worksheet by using a custom view (not a saved view) and see if that helps.
Re: Level 1 Model Builder: 13.9.3 Activity: Add Primary Grid to Volume Entry Page
I have exactly the same problem and can't find a solution even using a custom view. I also tried with the Level 1 Model Building Example to see if I made a mistake building my model through the activities but it is the same. The "allow editing button" is always grey and can't be activated, no matter which module I use (not only for REV02).