To be able to generate an Invoice Number the bill must be finalised.
When the bill or bill splits are sitting at Zero, the Finalise button will appear.
When clicking on Finalise, no further changes can be made and an invoice number is generated.
If there are any bill splits that have not had the Finalise button selected clicking the Check Out button at the bottom right will Finalise them automatically.
Clicking on View will allow you to print or send a copy of the bill with the invoice number now generated to the guest.