I'd be happy to help you how to get the invoice link in QuickBooks Online, @brownswoodworking.

You may need to enter a dummy email or your customer's email so that the option to get the link will be activated. This way, you can share it on social media with no sweat!

I've replicated your concern to show you why you're getting that error message:

As you can see, when you put an email in the text box, the option to get the link will be available.

Now that you're ready to share the invoice through social media, you may consider checking this article: Customize invoices, estimates, and sales receipts in QuickBooks Online. This contains tips and steps on how you can beautify your sales forms to get more sales!

Let me know if you have other questions. It will be my pleasure to help you further. Have a good one and wishing your business more profit this year onward!