Why the Sync Error occurs
This error occurs when using features in Bill.com that QuickBooks Online (QBO) Essentials subscription does not support, such as billable expenses and class tracking.
How to Fix the Sync Error
To use those features in Bill.com and sync to QuickBooks Online, upgrade to QuickBooks Plus. Contact Intuit to determine the subscription needed to sync with Bill.com based on the features needed.
Alternatively, do not use the features in Bill.com that are not supported by the QuickBooks Online Essentials subscription.