When using the Auto-numbering setting for Sales or Purchases and you have previously used an invoice number you will get the following error message which prevents the invoice being created.

The invoice number that you entered or auto-generated (x) has already been used. Please choose a different invoice number.

You shouldn’t get any error messages if (a) the number is available and (b) there isn’t an equal or higher invoice number being used in the system.

How to fix this

Find the latest/highest invoice number and then go to Settings > Sales and set the “Next invoice number” field to be the latest/highest invoice number you have used plus one. i.e. if you have issued sale number 345 then the number to enter in the “Next invoice number” field is 346.