There was a problem adding this invoice?

It's our priority to ensure you'll be able to add invoices in QuickBooks Online (QBO). , a-kennerley. 

First off, I'd like to make sure that we are on the same page so I can help you out with your concern. I'll need additional information about the error message you received when you tried to add an invoice in QBO. This way, I'll be able to provide an accurate solution to your concern.

In the meantime, we can perform some basic troubleshooting steps to make sure the issue isn’t browser-related. It’s possible that local internet cache files are the reason for this unexpected behavior. Let's start accessing your account using a different or private browser. 

You can use either of these shortcut keys:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

Once in, try to add the invoice again. If it works, I recommend clearing your browser's cache. Doing so will remove the historical data and access QuickBooks with a clean slate. You can also switch to a different supported browser.

I'm also adding this link to know more about customising your invoices: Customise My Invoices.

Let me know if you have other questions aside from adding invoices in QBO. I'm always happy to help you. Have a great day.