Welcome to the Community! It's great to see that you're taking advantage of all the features QuickBooks Online offers to manage your business. I can help you determine what's happening with this entry.
In QuickBooks Online, the email address field is optional for all entry types such as invoices and sales receipts, so you should be able to save them without having to enter an email. If the system isn't letting you save, this could perhaps be a performance issue caused by a build up of cache and other data within your web browser. I recommend trying the following browser troubleshooting steps, checking between each to see if you can now save the entry without a customer email address:
Reset your Internet or try on another computer or device
Please feels free to reply to this thread and elaborate further about the type of entry you're attempting to save. The more information we have, the faster we can help you resolve this!
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