I had a client report this issue recently which was preventing them submitting the VAT return to HMRC via the government gateway. The exact error is below

After some digging it seems the period key is passed down from HMRC when the obligations are retrieved therefore its strange that the error is produced when submitting the VAT return as it should be using the same period key.

I looked at the TX00300 table (VAT Return Header) and I can see the PeriodKey was blank for the VAT Return ID in question

I then checked a clients system who I know has managed to process their VAT return successfully and this showed a PeriodKey in the TX00300 as per below

Therefore I can only assume that after pulling down the obligations either the PeriodKey wasn’t written to the TX00300 table correctly or the client accidentally changed a date in the VAT Return window which blanked the PeriodKey. Then, when they then tried to submit the VAT return they received the “PERIOD_KEY_INVALID” error.

To resolve the issue I had the client delete and recreate the VAT return ensuring no dates were altered after selecting the obligation and this then worked fine. (I did want to work through the whole process with them so I could check the PeriodKey in the TX00300 at every stage however unfortunately they processed everything before I had chance i.e. I wanted to check the PeriodKey value after getting the obligations, and then check it again after calculating and then prior to submitting)

In was a simple fix in the end but it was interesting piecing things together.

Thanks for reading.