Thank you for bringing this issue to our attention. Let's ensure to add an invoice to your customer successfully in QuickBooks Online, (QBO).
Before we begin, could you please let me know if you've encountered any error messages while creating invoices? This information will help us identify the necessary steps to resolve the issue. If you're using a web browser, I suggest opening your QuickBooks account in incognito mode to help isolate any browser-related issues.
Here are the shortcut keys:
Mozilla Firefox and Microsoft Edge: Press Ctrl + Shift + N
However, if your using an app, I suggest clearing the app's cache. By doing so, you can remove historical data and access QuickBooks with a clean state.
Here's how:
iOS
Go to Settings, then tap General.
Click iPhone Storage, then tap the QuickBooks Online app.
Select Offload App.
This will free up the storage without deleting your app's documents and data.