Error ORA-02270

While upgrading to the 12.5 version I've got the error:

Upgrader Error Messages:

TASK_ID     : 2

TASK_NAME   : Pre Upgrade Schema

BATCH_NUM   : 2

ERROR_TYPE  : FATAL

ERRANT_STMT : ALTER TABLE DATAUSER_UNIQUELOGINS_STG ENABLE CONSTRAINT DATAUSER_UNIQUELOGINS_STG_FK1

ERROR       : ORA-02270: no matching unique or primary key for this column-list

Does anybody know what does it mean and how to fix it?