I can see the urgency of getting this resolved, @itchydog.
There's currently an ongoing issue with the Invoice being unavailable when trying to view. Rest assured, our Product Engineers are working to fix it as quickly as possible. While they do, I recommend reaching out to our Customer Care Team, so you'll be added to the list of affected users and receive email updates.
To reach them, click the ? Help button at the top-right corner and select Contact Us to talk with a live agent. Ensure to review their support hours to know when agents are available.
If you want to add customization on your sales forms in the future, you can check this article for a guide: Customise invoices, estimates, and sales receipts in QuickBooks Online.
I appreciate your patience while we work for a fix. If you have additional assistance while managing your QuickBooks account, let me know. I'll be around to provide further assistance. Have a nice day.