Hello mugelbbub, thanks for dropping by on this thread 

This is a limitation of FRS in QuickBooks currently when the transaction has a standard 20% VAT code applied and is dated within a filed period. If you do need to correct a transaction, you can reverse the journal and then create as new. 

We have passed feedback onto our Product team so that this can be considered as an area of enhancement in the future. If there is anything else that we can help you with, please do get in touch.