Error 'Cannot find a valid GL treatment from revenue recognition rule XXX for the revenue schedule. A GL treatment must have matching legal entity and currency with the revenue schedule.'

Salesforce Billing uses the "Unique Id" (blng__UniqueId__c) field on the GL Treatment for matching a valid GL Treatment to the Revenue Schedule. The Unique Id populates the Id for the GL Rule + Legal Entity (+ Currency Code, if using Multi-Currency) automatically. If it has the wrong Id fields or is null, Salesforce Billing cannot match the GL Treatment.

To correct, clear out one of those three lookups on the GL Treatment (either GL Rule, Legal Entity, or Currency Code), save, then re-populate, and save again. It should update the Unique Id with the correct Id field.