Insert query error

I am getting the following error trying to run the below query. Any suggestions on what I am doing wrong? Thank you, Something went wrong while inserting your store visitCannot add or update a child row: a foreign key constraint fails (store.posts, CONSTRAINT posts_ibfk_1 FOREIGN KEY (post_store) REFERENCES stores (store_id) ON DELETE CASCADE ON UPDATE CASCADE) $sql = "INSERT INTO posts(market, type, post_store, post_tptest, post_content, post_by, post_date) VALUES('" . mysql_real_escape_string($_POST['market']) . "', '" . mysql_real_escape_string($_POST['type']) . "', '" . mysql_real_escape_string($_POST['post_store']) . "', '" . sha1($_POST['post_tptest']) . "', '" . mysql_real_escape_string($_POST['post_content']) . "', " . $_SESSION['user_id'] . ", NOW())";