combination of "WHERE NOT EXISTS" & "SELECT 1 FROM" not working

I've an query working in SQLite but it's not working in MySQL.

I'm using: PHP, PDO, SQLite & MySQL

Here is the query:

INSERT INTO mytable (id, name, tag, timestamp)
    SELECT 11, 'Legio', 'LR', 1234567
        WHERE NOT EXISTS (
            SELECT 1 FROM mytable 
                WHERE id = 11 AND name = 'Legio' AND tag = 'LR' AND timestamp = (
                    SELECT max(timestamp) FROM mytable WHERE id = 11))

The result is an error message:

Syntax error near 'WHERE NOT EXISTS ( SELECT 1 FROM mytable WHERE id = 11 AND name ' at line 3

What is wrong with this query in MySQL???