Hello there, @Tmcnair.
Let's make sure you'll be able to send an invoice without any problem in QuickBooks Online (QBO).
To check if the problem is browser and cache related, first, you can access your account using a private browser or another supported browser and try sending an invoice from there.
You can use these shortcut keys to access a private browser:
If you're able to send an invoice using a private browser, you have to clear the cache of the regular browser you're using. For the steps, you can check it here: How do I clear my browser cache and temporary Internet files?.
However, if the problem continues, I recommend checking this with our Support Team to investigate this further. For the support's contact information, you can check it here: Contact the QuickBooks Online Customer Support team.
You can also visit our help articles for your reference in the future: Help articles for QuickBooks Online.
Feel free to add a comment below if you have any other questions about sending invoices. I'm always happy to help.