Hello there, @sharaththanneer-.
I'd be happy to help you how to save the Invoice without putting an email address.
You can select the drop-down button beside the Save and send button then, choose Save and close. I've added a screenshot for your reference:
I've included this article to learn more on how to manage your invoice: Void or delete transactions in QuickBooks Online.
You may read through this reference page in case you want to make your sales forms more appealing: Customise invoices, estimates, and cash memos in QuickBooks Online. This also contains sub-articles on how to create progress invoices and customizing logos.
I'll be here if you have other questions. Please don't hesitate to let me know in the comment section. Take care.