ERROR: 'You can't edit this transaction as it has an associated payment or refund.'

You see this error when you saved the payment on the supplier's account first, then allocated it to an invoice later.

To edit the payment:

1. Break the supplier allocation.
2. Edit the supplier payment.

ERROR: 'Included on a VAT Return.'

You see this error when you use the Cash Accounting VAT scheme, because supplier payments appear on the VAT Return.

If the payment is on a draft VAT Return:

1. Delete the VAT Return.
2. Edit the supplier payment.

If the payment is on a submitted VAT Return:

Reverse the payment, then re-enter it. You can only reverse a payment that doesn't pay an invoice. So you may first need to unallocate the payment from the invoice.