Once an Invoice is Processed or Cancelled, it is unable to be amended.
The Invoice will need to be reverted to the Reconciled state and re-processed.
To revert a Processed Invoice, please follow this link, Revert a Processed Invoice.
To revert a Cancelled Invoice, please follow the below steps.
From 'Dashboard', go to [Documents] [Invoices]. Search for the Invoice, in the 'Search for Invoices' field.
Click the 'Reopen' button at the right bottom corner of the Invoice as shown below.
3. The Invoice will revert to the Reconciled Invoices folder.
4. Once the Invoice is in the Reconciled column, the Invoice can now be amended, and the Account code added.
However, if the Account code is not showing, either the Account code has not been created or not enabled to the department. Double-check the Account codes in the Organisation's Accounts Payable Hub. Please check out this article for more information, Add an Account code to Accounts Payable hub. (Note - This link will also show you how to add the code to your Account).