When a company has been deleted and is in the ConnectWise Recycle Bin, errors will result when the company billing contact is attempted to be returned from ConnectWise as part of the sync process.
Company Recycle Bin:
Invoice to Company in the Recycle Bin: 
To resolve this issue, you can either
a) Restore the company from the Recycle Bin
1.Select the company in the Setup Table > Mass Maintenance > Company Recycle Bin
2. Select "Restore Company" from the action menu
b) Set the unposted record to an alternative (active) company.
1. Open the Records in ConnectWise
2. Select an active Company
3. Save / Close the records so the records will re-present in the Accounting Interface.
Records that are posted to your accounting package will require an active and selected billing contact for the company that is defined in the Bill To of the record being posted.