ORA-39420
transportable tablespace 'string' clean offline SCN mismatch [string] [string]
Cause
The tablespace clean offline SCN changed during a transportable Data Pump export operation. Tablespaces in the transportable export tablespace set are required to be and to remain in READ ONLY mode for the duration of the operation. During the transportable export operation, a tablespace within the transportable tablespace set was taken out of READ ONLY mode. This resulted in the tablespace clean offline SCN being altered. As a result, the tablespace is incompatible with the current transportable export operation.
Action
ALTER the tablespace to be READ ONLY and then retry the operation.