For TAD in purchasing how can we get Bill to BU name as source to create mapping rule

Summary:

For TAD in purchasing how can we get Bill to BU name as source to create mapping rule.

Content (please ensure you mask any confidential information):

Based on the Bill to Business unit, We need to update a Segment for Accrual Account. but what the correct source to get the bill to BU as input source in Purchasing Mapping set . We have tried using the source XCC_BILLTO_BU_ID in Purchasing mapping set and used this mapping set in Purchasing Account rule and assigned to the Accrual Account segment in TAB but it gave below error while validating the TAD