If I understood well, you’re trying to add the column ln_classified_st to the table florabank_online.provision.
Besides specifying the column may not be null you also try to add the same constraint explicitly.
I guess you should try:

ALTER TABLE florabank_online.provision ADD ln_classified_st VARCHAR2(3) NOT NULL;

Not specifying a default value, this default value is automatically NULL.
Specifying NOT NULL you create the desired constraint.

If you want a named constraint then:

ALTER TABLE florabank_online.provision ADD (
  ln_classified_st VARCHAR2(3)
                   DEFAULT NULL
                   CONSTRAINT DF__interest_provision_ln_classified_st NOT NULL
)

If you want an unnamed (or system-generate name) constraint then:

ALTER TABLE florabank_online.provision ADD (
  ln_classified_st VARCHAR2(3)
                   DEFAULT NULL
                   NOT NULL
)

Note: In Oracle, '' is identical to NULL so DEFAULT '' is the same as DEFAULT NULL.

Note 2: The first- and last-brackets () in the statement are optional when you are adding a single column.

Note 3: If you are running the statement using EXECUTE IMMEDIATE or via a third-party language (such as Java, Python, C#, etc.) then the invalid character is probably the ; at the end of the statement. You should not include that in the statement. If you are running it as part of a script (i.e. in SQL*Plus, etc.) then you need statement terminators (i.e. ; or / on a newline) between statements but the trailing ; is not part of the SQL statement.