'You cannot record entries prior to your Accounts Start Date of DD/MM/YYYY.'
This error message is thrown when you are trying to publish an invoice that is dated as prior to the date of your accounts software's setup.
To clear this error, you will need to do the following:
Unpublish the invoice
Change the date of the invoice to a date after your accounts software was set up
Republish the invoice
Please note: If that means the date will be incorrect when it is then published to your accounts software, you can amend it in the software afterwards.