Oracle SQL error "ORA-00984: column not allowed here" when trying to replace a column - sql

Question

This question is not a duplicate from this one because even if the error messages are equal the answers there do not apply to my case.
I need to change a previous PK Id column defined as VARCHAR(36) NOT NULL to an incremental integer value. I'm trying to write a script to do this but when I run the alter table statement it fails with the error on the title.
The table is previously defined as:
CREATE TABLE journal_messages(
ID VARCHAR(36) NOT NULL, -- column to be changed
MESSAGE VARCHAR(2048) NOT NULL,
MESSAGE_TYPE VARCHAR(30) NOT NULL,
MSG_DATE TIMESTAMP NOT NULL,
MODULE_CODE INTEGER NOT NULL
);
ALTER TABLE journal_messages ADD (CONSTRAINT journal_messages_pk PRIMARY KEY (ID));
The script I'm running is:
DELETE FROM JOURNAL_MESSAGES;
ALTER TABLE JOURNAL_MESSAGES DROP COLUMN ID;
CREATE SEQUENCE journal_messages_seq START WITH 1;
ALTER TABLE JOURNAL_MESSAGES ADD (ID NUMBER(10) DEFAULT journal_messages_seq.nextval NOT NULL); -- error happens here
ALTER TABLE journal_messages ADD (
CONSTRAINT journal_messages_pk PRIMARY KEY (ID));
When I try to create a trigger it to update the incremental, it fails with SQL Error [4098] [42000]: ORA-04098: trigger 'TRG_SEQ_JOURNAL_MSG' is invalid and failed re-validation when I try to insert a new tuple:
ALTER TABLE JOURNAL_MESSAGES DROP COLUMN ID;
ALTER TABLE JOURNAL_MESSAGES ADD (ID NUMBER(10) NOT NULL);
create or replace trigger trg_seq_journal_msg
before insert on journal_messages
for each row
begin
:new.id := journal_messages_seq.nextval;
end;
/
INSERT INTO JOURNAL_MESSAGES (message, MESSAGE_TYPE, msg_date, MODULE_CODE) VALUES ('test', 'alteration', CURRENT_TIMESTAMP, '10');

Answer 1

Umm ... not like that, but like this:
once the table is empty, you don't have to drop the column - just modify its datatype
use a trigger to automatically set IDs value
If you were on 12c, you could have used identity column.
SQL> CREATE TABLE journal_messages(
2 ID VARCHAR(36) NOT NULL, -- column to be changed
3 MESSAGE VARCHAR(2048) NOT NULL,
4 MESSAGE_TYPE VARCHAR(30) NOT NULL,
5 MSG_DATE TIMESTAMP NOT NULL,
6 MODULE_CODE INTEGER NOT NULL
7 );
Table created.
SQL> delete from journal_Messages;
0 rows deleted.
SQL> alter table journal_messages modify (id number(10));
Table altered.
SQL> CREATE SEQUENCE journal_messages_seq START WITH 1;
Sequence created.
SQL> create or replace trigger trg_bi_joumes
2 before insert on journal_messages
3 for each row
4 begin
5 :new.id := journal_messages_seq.nextval;
6 end;
7 /
Trigger created.
SQL>
[EDIT: after reading your comment and saw your edition]
That still works OK - I literally copy/pasted your code and got this:
SQL> ALTER TABLE JOURNAL_MESSAGES DROP COLUMN ID;
Table altered.
SQL> ALTER TABLE JOURNAL_MESSAGES ADD (ID NUMBER(10) NOT NULL);
Table altered.
SQL> create or replace trigger trg_seq_journal_msg
2 before insert on journal_messages
3 for each row
4 begin
5 :new.id := journal_messages_seq.nextval;
6 end;
7 /
Trigger created.
SQL> INSERT INTO JOURNAL_MESSAGES (message, MESSAGE_TYPE, msg_date, MODULE_CODE) VALUES ('test', 'alteration', CURRENT_TIMESTAMP, '10')
2 ;
1 row created.
SQL>
As you can see, everything seems to be just fine. Try the following: recompile the trigger and show errors (if any; if so, please, post them here):
SQL> alter trigger trg_seq_journal_msg compile;
Trigger altered.
SQL> show err
No errors.
SQL>