I don't seem to be able to send an invoice on my mobile app anymore, keeps saying there is an issue and contact us if problem persists.

I'll help you fix this, stevengommltd-gm.

I've checked our records and there's no reported case about this one. Odd behavior when sending an invoice in the mobile app is commonly caused by a corrupted cache in the system. To rule this out, try clearing the accumulated app data by following the steps below: 

For Android devices:

1. Head to your phone's settings,
2. Choose Apps (varies by device).
3. Select QuickBooks Online from the application list.
4. Go to the Storage section, then tap Clear data.

For iOS devices:

1. From your phone's settings, tap Settings.
2. Choose General, then select iPhone Storage.
3. Select QuickBooks Online from the application list.
4. To free up the storage without deleting the documents and data for the app, select Offload App.

Once done, try sending your invoices again. If the error persists, you may need to uninstall and then reinstall the application. Alternatively, you can log in to your QuickBooks Online company using a web browser. Then, send your sales transactions from there to further isolate the issue.

I also encourage reading this article to learn how to personalize the invoices that you send to your customers: Customize sales forms.

Feel free to update us after reviewing the entries. I want to make sure this is resolved and I'll be right here if you need further help. You have a good one.