Help with syntax error

I am trying to run a sql script in a mariadb version 10.3.38-MariaDB-cll-lve:

SET
@s := '2021-07-12 12:00:00';

SET
@e := '2021-07-12 12:30:00';

INSERT
	INTO
	appointments
        (client_id,
	service_id,
	starts_at,
	ends_at) 
    SELECT
	(
	SELECT
		id
	FROM
		clients c
	WHERE
		c.email = 'some@email.com'),
	(
	SELECT
		id
	FROM
		services s
	WHERE
		s.name = 'ANY'),
	@s,
	@e
WHERE
	NOT EXISTS 
        (
	SELECT
		id
	FROM
		appointments
	WHERE
		(starts_at <= @s
			AND @s < ends_at)
		OR (starts_at < @e
			AND @e <= ends_at)
		OR (@s <= starts_at
			AND starts_at < @e)
            )
  
;

getting this error:

SQL Error [1064] [42000]: (conn=5686202) You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'WHERE
	NOT EXISTS 
        (
	SELECT
		id
	FROM
		appointments
	WHERE
...' at line 26
  (conn=5686202) You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'WHERE
	NOT EXISTS 
        (
	SELECT
		id
	FROM
		appointments
	WHERE
...' at line 26
  (conn=5686202) You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'WHERE
	NOT EXISTS 
        (
	SELECT
		id
	FROM
		appointments
	WHERE
...' at line 26

Could you give me a hint or any solution?