To validate your recruitment plan, you have to SAVE it first,
then click on 
VALIDATE in the right horizontal
toolbar. (VALIDATE is only active when the recruitment plan has
been saved). As soon as the recruitment plan has been validated
the inscription of the VALIDATE button changes to 
INVALIDATE.
To edit a valid recruitment plan, you have to INVALIDATE first
and then click on edit.
Always remember to validate your recruitment plan when you have
finished making changes.