I can no longer see the add invoice anywhere in the mobile app

Let me help you sort out this issue, Kev.

A corrupted cache in the system commonly causes these runtime errors and unresponsive interfaces. To fix this, we can refresh your data to see the latest available information in your mobile app. To do so, follow the steps below:

For iOS:

1. Go to Settings, then tap General.
2. Tap iPhone Storage, then tap the QuickBooks Online app.
3. Tap Offload App.
   This will free up the storage without deleting your app's documents and data.

For Android:

1. Go to Menu ☰, then tap More Options ⋮.
2. Tap Settings, then Refresh Data.
3. Tap YES to confirm.

Furthermore, if you plan to get paid for the products and services you see, send your customer an invoice. To learn how to manage and personalize your invoicing preference. I've added this article for more details: Create and Send An Invoice in QuickBooks Online.

You can always post your comment below if you have any other concerns or follow-up questions about invoices. Remember, I'm always here to help you work towards a resolution to ensure you'll be able to view invoice information.