Payment sending to Xero by Xeroom failed. The Invoice needs to be sent to Xero first.

Xero cannot receive the invoice payment if the invoice has not first been created by posting the order from WooCommerce since it needs to match up the payment to the invoice.  If you try to do this then this message will be given.  

If you wish to send prepayments on account these can be handled by using the Funds on Account plugin which Xeroom is customised to work with.

To send the invoice first either set the automated send trigger or else send them individually by using the button in the bottom right of the order or else as a Batch Send.

In your WooCommerce order management screen select which orders you wish to send  and then select the relevant Bulk Acttion.  This creates a cron job which then executes slowly to avoid hitting the Xero API call rate limit and will run until cancelled or completion.