You are receiving the below error when trying to publish an invoice.
Invoices with VAT summary lines cannot be marked as paid in companies on the VAT Cash Accounting (UK) VAT scheme.
Sage 50 UK integrated companies.
Purchase invoices marked as paid.
For companies on the cash based schemes mentioned in the error message, payment transactions for invoices marked as paid are now posted with the same tax code as the invoice (for the other schemes payments are posted with a value of T9). There are now also added restrictions for invoices for companies integrated with Sage 50 UK and using these VAT schemes.
Invoices which have line items/tax summary items and mark as paid enabled for that invoice will throw an error when you attempt to publish them to Sage.
You can clear this error by doing the following: