Did you get a resolution to this issue?  One of our customers is receiving the same error

I’m not 100% sure, but I think the Max Liability Date is a calculated field in the background, which is why you can’t see it. 

Italian tax requirements are for tax transactions to be numbered progressively based on the liability date, therefore you can’t create tax transactions for liability dates earlier than the last one you posted (i.e. you can’t create a new tax transaction for 2nd January if you’ve already put one in for 3rd). So the max liability date is set to be the latest date a tax transaction has been created for, and is updated as new transactions are entered.

Reference to Italy Country Solution may provide better clarity than my ramblings here :D

Thanks Carl.  This issue was reported to IFS and a correction was done of the existing erroneous tax series numbers via a DR script.  I understand there is a service update that the customer is deploying which will prevent the issue reoccurring.  

If you haven’t already - check the highlighted parameter in Tax Ledger.

This should allocate the tax transaction when you fetch tax ledger information, as opposed to posting the voucher, which may help prevent the error occurring at source in the future.