Hi youssef.
Go on your current works order - click View Tab - Picking list (new form will open) see if there's any open picking lists to be posted. You need to make sure you consume/complete your materials prior RAF. One of the reasons why you are getting this error.
If you are unsure if you consumed all materials or not, on the same form select from top action pane Picking list drop down menu - Create lines with proposal - Remaining Quantity.
As well check if you marked all your operations as completed.
Once all checked and posted, there shouldn't be an issue for you to RAF your works order.
Let us know if you are still struggling
Regards,
V