Hi AJM0517.
Thanks for getting in touch.
When creating an invoice on QuickBooks Online you shouldn't need to enter an email address unless you are wanting to send this out to your customer. If you create the invoice and then click 'Save' or 'Save and Close' the invoice should save to your QuickBooks without needing an email address. The only thing you would need would be customer name. I have just tested this on my own QuickBooks Online account and it has allowed me to save the invoice without including an email address.
If you are getting an error message would you be able to send a screenshot of the page that is showing when you try to save without an email address.
Let me know and I will be back in touch shortly :)
