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 Xero. For example, if within Xero you've deleted an item or account code, tracking 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 Xero. 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 Xero 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.