There are 3 reasons for which your user account will not be able to make changes in Prima:
1. The record or report is locked by another Prima user (you can see this at the very bottom of the screen, if locked it will say by which user)
2. The Status of the report prevents any changes being made (e.g. a Sales Order which is Status Delivered, you may not add any more lines to this order)
3. The user does not have permission to make the required changes (your Prima Administrator can view user permissions via the User Administration option in the Administration)