When trying to finalise the September BAS in QBO I get the following error 'The sum of split amounts must be equal to 0'. Does anyone know what this is and how to fix?

Welcome to the QuickBooks Community, @sheffer2.

The error message is usually caused by a negative value on your BAS. Once this is taken care of, you should be able to mark it as lodged. 

At this time, we're receiving several reports from other users experiencing the same prompt when preparing the September data. Our engineers know and they're working to find a permanent fix to this unusual behavior. 

In the meantime, I recommend giving our Customer Care Team a call to add your account to the list of affected users. You'll receive an email notification once an update is available. 

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1. Click the Help menu at the top.
2. On the pop-up, click Contact us.

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Hit the Reply button and let me know how the call goes or if you need anything else. I'm always here to help. Have a good one!