Quickbooks Online Sync Error | Feature Not Supported Error | Feature Not Supported Error : We're sorry. This feature is not included in your QuickBooks Online Essentials subscription.

If the Billable field is being used on the item or expense lines, please note that this feature is only supported for QuickBooks Online Plus or Advanced plans.

On your current plan (Essentials), QuickBooks does not support the Billable field for synced bills, which is why the sync is failing.