If an invoice is in “Not Submitted” status, this simply means the invoice has been saved, but has not yet been submitted to the invoice reviewer for approval. The invoice reviewer cannot view invoices with a Not Submitted status.
The most common reason an invoice would be in this status is because the invoice is still in the process of being created. An invoice with this status can be edited freely without any restrictions. For example, you can change the date, change the client, and add and remove invoice items.