Account Rule error "You can't use a mapping set, which uses input source Invoice Line Flexfield Attr

Summary

Error in Account Rule when using Invoice Line Flexfield Attribute as per 20C feature Use Additional Subledger Accounting Sources in Cost Management

Content

We would like to use the invoice line flexfield attribute to drive our COGS account and the 20C feature "Use Additional Subledger Accounting Sources in Cost Management" below in blue indicates this is possible.

We'd setup a mapping set XYZ using "Invoice Line Flexfield Attribute 1" then in the Account Rule change the mapping set to XYZ but the error  "You can't use a mapping set, which uses input source Invoice Line Flexfield Attribute 1 that isn't assigned to the event class Trade Cost Accounting. (XLA-871146)" appears.