1. Run the Diagnostic Tool for the Sync Object in Error
A sync object could end up in an error state for various reasons, most likely due to dependencies missing in QuickBooks. For example, if within QuickBooks you've deleted a product/service code, class category or something else in which your Bnbtally rules are relying upon to create the respective invoice, then the relevant sync objects will return an error.
If you select any individual sync object which happens to be in an Error state, you will notice that there's a "Run Diagnostic" button that will check all aspects of the sync object creation and see if there's any missing dependencies in QuickBooks. From there you can make the appropriate changes to your Bnbtally settings for the listing. Once your changes are made, you can then rollback the sync object in error and recreate it with the new working settings.
2. Rollback and Recreate the Sync Object in Error
Although rarely, a sync object error can happen when QuickBooks is under temporary maintenance and didn't completely process the sync object at the moment Bnbtally created it. If you have a sync object in error but the "Run Diagnostics" utility does not provide further reason for the issue, sometimes simply rolling back the sync object and recreating it will resolve the issue.
3. Disable Conflicting QuickBooks Settings
If after rolling back and recreating the sync object the error still persists then there may be a particular setting within QuickBooks that may be conflicting with the invoice/payment creation process. For example, within QuickBooks Online settings, under "Advanced Settings", look for the setting "Automatically apply credits" and make sure this is disabled. Although not always, this setting can cause conflicts with payments to invoices for any customers that have other existing payments, overpayments or credit memos.