This issue occurs when the opportunityID field for invoices in Quickbooks is empty.
The opportunityID is not getting written back to Quickbooks because of the “valid closing date password” error from Quickbooks.
When the mapping is as shown in the above image, the receivables will not get linked to salesforce when we encounter the “valid close date password” error with the opportunityID field being empty in Quickbooks.
Navigate to InvoiceToReceivables upsert mapping.
Under the AVSFQB__Invoices__c (Target) mapping finds the opportunityID field as shown in the image below.