Ensuring you're able to save your invoices is our top priority, berenfryer.
The QuickBooks Online application's stored data may be full. That's the possible reason why you encountered an error when saving invoices. To fix this, let's clear the app's cache. It helps restore to its default settings.
In the QuickBooks Online app:
Android - Select the menu at the top right and choose Settings. Then select Refresh Data and choose Yes.
iOS - Select the menu at the top right and then choose Help & Feedback. Then select RefreshData and Refresh.
In your Phone Settings:
Android - From your phone's settings, select Apps (varies by device). Choose the QuickBooks app from the list and select Storage. Select clear data.
iOS - From your phone's settings, select Settings, then, click on General and choose iPhone Storage. Select an app that's taking up a lot of space. To free up the storage without deleting the documents and data for the app, select Offload App.
Aside from that, you can also create and save transactions via browser. I'll show you how.