On phone and iPad app when trying to save an invoice it says: Unable to save We’re sorry-there was a problem. One of items associated with this entity is no longer valid

Hi there, ric-guernseytree.

Thank you for visiting the QuickBooks Community. I have some instructions to share so you can get past the error you encountered when trying to save an entry. 

There are times when your QuickBooks Online mobile app has a lot of data which may cause you to receive an unusual error message. To fix this, I recommend performing some troubleshooting steps by resetting the app data or clicking the Refresh Data option. This refreshes the data to ensure that you're able to save the invoice without any error and this will also help us clear out the stored files which impact and causes problems in the system.

Here's how to reset the app:

1. Click Help on the navigation bar at the top.
2. Select Reset App Data.
3. Allow a moment for the process to complete, and then close the app.
4. Be sure to right-click the app and Run as Administrator when re-opening.

Follow the steps below to refresh data:

1. From your iPhone's settings, click Apps(varies by device).
2. Choose the QuickBooks app from the list.
3. Go to Menu ☰.
4. Select Help & Feedback.
5. Look for Refresh Data.
6. Tap Refresh.

However, if the problem continues after performing the steps above, I suggest uninstalling and reinstalling the app to open your data on a clean slate.

Please refer to this article to see a table list that contains different features that are supported on mobile devices: Compare mobile app features.

Let me know how it goes by leaving a reply below. I'm here to help you further with syncing your entries in QuickBooks Online.