This typically occurs when the Bill.com Sync Preferences aren't complete. 

1. In Bill.com, hover over the gear icon, and then click Settings
2. Under Sync, click Preferences
3. Make sure all required fields have been set. 
4. Click Save
5. Sync again

For more information on Sync Preferences, please refer to the sync setup guide that fits your account setup and workflow. Learn more here: 

Sage Intacct Sync User Guide