Invoice # cannot be empty

Invoice # cannot be empty

This message means that Xero is being asked to create an invoice without an invoice number.  For some reason the number is not being created or is being interfered with by another plugin or integration feed before it reaches Xero.  

The invoice no is generated by Xeroom by either taking the next number in the Xero sequence or by picking up a customer no from another plugin such as the WooCommerce Sequential Order Number plugin or one of the many PDF invoice no plugins https://support.xeroom.com/support/solutions/articles/77000538064-using-pdf-invoice-wc-sequential-numbering-or-other-plugins-for-xero-invoice-numbers.  

Xeroom may further modify it as per your settings - see below:

If the Xero Numbering is selected then this is the simplest case and if you are getting this error with this selection then it means there is a conflict going on so please check your WooCommerce debug log - for details on how to do this https://support.xeroom.com/support/solutions/articles/77000535598-how-to-get-help-quickly.

If you are using the next option with a start number that is below the current number in Xero then it means the numbers you are trying to use are already used and cannot be used again since each invoice has to have a unique no in Xero.