Unless there are other hints or error messages, click on "Display all" for more information:
- Alerts: The invoice can be send even if there are shown other hints.
- Error messages: They have to be cleared before sending an invoice. If you're not able to solve the error yourself, then use the contact form and ideally add a screenshot of the error message.
By clicking on the description of the individual hint or error message you will get directly referred to the concerned field or box.