Why does SQLite give Cannot create instead of trigger on table

With this table

create table FOLDER
(
    _ID integer primary key autoincrement,
    NAME text not null,
    PARENT integer not null,
    DELETED integer,
    constraint VALUEOF_folder_deleted check (DELETED == 1 or DELETED isnull) on conflict abort,
    unique (NAME, PARENT) on conflict abort
);

I wanted to replace an insert where the NAME PARENT combination exists and DELETED is set to 1 with a replace setting the DELETED field to null.

I tried this trigger:

CREATE TRIGGER REPLACE_INS_folder instead of insert on FOLDER
when exists (select 1 from FOLDER where NAME == new.NAME and PARENT == new.PARENT and DELETED == 1)
begin
    update FOLDER set DELETED = null where NAME == new.NAME and PARENT == new.PARENT;
end;

but received: Error: cannot create INSTEAD OF trigger on table: FOLDER

Initially I had a syntax error in the trigger, but received the same error, suggesting that there is something more restrictive going on. The diagram in this documentation https://www.sqlite.org/lang_createtrigger.html suggests my trigger is valid. However, the second part of the text A trigger may be specified to fire whenever a DELETE, INSERT, or UPDATE of a particular database table occurs, or whenever an UPDATE occurs on on one or more specified columns of a table. makes me wonder if WHEN is only allowed with update triggers.

Note this is a trigger on a table, not on a view.

Solution

The documentation says:

Triggers may be created on views, as well as ordinary tables, by specifying INSTEAD OF in the CREATE TRIGGER statement.

This is rather misleading. What this sentence is actually trying to say is that

on normal tables, you can use only BEFORE and AFTER triggers, but
on views, you can use only INSTEAD OF triggers.

You cannot use INSTEAD OF triggers on a table. Change it into a view:

CREATE TABLE FolderTable(...);
CREATE VIEW Folder AS SELECT * FROM FolderTable;
CREATE TRIGGER ... INSTEAD OF INSERT ON Folder ...;

(You then also need INSTEAD OF UPDATE/DELETE triggers.)