It's vital you're able to email invoices to customers so you can get paid. I'd be happy to show you how to fix that error message.
First, I'd like to start by doing some troubleshooting to determine if this isn't a browser related issue. When was the last time you cleared cache and cookies on your browser? Doing this will help fix most issues the site may be having. Here's an article which shows you how to do that: How to clear Cache and Cookies. Give this a try and if you're still having issues, I recommend contacting our support team using this link.