It is a required field when creating invoices in QuickBooks Self-Employed, DharshananZ.
QBSE's invoicing feature is simplified that's why all the information asked needs to have a detail in it. I have sent this as feedback to our Product Development team and they might include an option to be able to save without an email address.
Meanwhile, we can enter a dummy or your own email address for you to be able to save it.
You can check out this article that you can check out on how to manage your sales:
I'm here if there's anything else that I can help you with.
