If none of these fixes the problem, I suggest contacting QuickBooks Online Support. They're equipped with tools to further investigate this.
You can also share a screenshot of your screen when trying to create the invoice. It'll help us further investigate its cause.
I appreciate your business and want to make sure that you're completely satisfied with our service. If there's anything else I can do to assist you, please don't hesitate to let us know. I'm always here to help and committed to resolving any issues you may have.