It appears that you are trying to set a column to be the identity column, using the ALTER TABLE command. Oracle, however, tells you that you can't do that.
An example:
Create a table:
create table test (id number);
This command raises the ORA-30673:
alter table test modify id number generated by default on null as identity;
You should create a new identity column:
alter table test add id_new number generated by default on null as identity;
Then copy data into a new column, if necessary:
update test set id_new = id;
Drop the old ID column:
alter table test drop column id;
Rename the new column to the old name:
alter table test rename column id_new to id;
Expect problems if the old ID column was a primary key, used in enforcing foreign key constraint(s).