PLS:00201: identifier 'FND_FILE.LOG' must be declared

Solution steps.

 SQL> show user;

USER is "APPS"

SQL>GRANT execute ON APPS.FND_FILE TO cust_schema;

SQL> show user;

USER is "cust_schema"

SQL>

SQL> CREATE OR REPLACE SYNONYM FND_FILE FOR APPS.FND_FILE;

Synonym created.

I hope issue has been resolved.