Add customer reference date to Customer Invoice Form Professional Services (SDBIL_CI_PROF_SRV_XX)

Dear Community,

we would like to add the customer reference date from a customer project to the Customer Invoice Form Professional Services (SDBIL_CI_PROF_SRV_XX) form. While we were able to include the customer reference from the header data using the field "$.PurchaseOrderByCustomer", we get the following message when including the customer reference date ($.GroupItems.BillingDocumentGroupItemNode[*].SubTotal.BillingDocumentGroupItemSubTotalNode[*].Items.BillingDocumentItemNode[*].CustomerPurchaseOrderDate): Code 20032: Direct binding references to multiple levels of repeating data may not produce preferred results. Form may require relative binding references for repeating subform containers.

How do we correctly include the customer reference date of a customer project in the form?

Thank you very much and best regards Bianca