There are some issues that will stop the invoice from being able to be sent. When these occur, the user will be told what the issue is and how to fix it:
No Accounting Period
If the firm has not set an accounting period they will need to set it in Billing Settings before sending invoices
Invoice Date not open in an Open Accounting Period
If the invoice date is not in an open accounting period, they will not be able to approve or send the invoice. They will need to select an invoice date in the current accounting period
If the invoice total = a negative value. The user will need to resolve the issues and retry
If the user tries to send an invoice with missing details they will be told which details are missing and need to add them. If they try to approve, they will just be warned.
Once you've resolved the issue, you can then send your invoice to your client.