VAT Process

Hi We are new to SAP Business One and unfortunately do not get much assistance from our partners! The financial controller has to process a HM Customs and exercise certificate. She is trying to post a journal to the purchase VAT account which as its a control account the system will not allow her too. That's not the correct way to process the C79 return but I cant find any help on what the correct process is. This has now become urgent. Can anyone advise what the process is for VAT returns so it is posted to the correct GL accounts I cannot find anything searching on google or the SAP Help

Thanks