Oracle Message DRG-10503 - table string cannot be indexed because it has no primary key

ERRORtable string cannot be indexed because it has no primary key
CAUSEa textkey needs to be a single column unique key whose length does not exceed 64 bytes. no such key was defined as an unique index or declared as a primary or unique key constraint
ACTIONcreate a suitable textkey for the table or specify it explicitly in this procedure call