How to get the second string in the ORA-02296 error to show something meaningful
Provide a name for the constraint:

SQL&gt; create table monkey(tail_count number);

Table created.

SQL&gt; insert into monkey values (null);

1 row created.

SQL&gt; alter table monkey modify tail_count not null;
alter table monkey modify tail_count not null
*
ERROR at line 1:
ORA-02296: cannot enable (ANIMAL.) - null values found

SQL&gt; alter table monkey modify tail_count constraint tail_count_not_null not null;
alter table monkey modify tail_count constraint tail_count_not_null not null
                                                *
ERROR at line 1:
ORA-02296: cannot enable (ANIMAL.TAIL_COUNT_NOT_NULL) - null values found