ORA-11426
Use of flashback query is not yet supported by the container.
Cause
An attempt is being made to execute a SQL statement using an AS OF TIMESTAMP clause. A flashback query clause specifying a timestamp is not supported by the container from which it is being executed. Remote refreshable clones and Oracle Data Guard pluggable databases are examples of container types that have this limitation.
Action
Changing the SQL to use AS OF SCN may help. It will either give the correct results or raise ORA-08181, which may be due to the lack of support by the container type.