If you just want to  redirect to checkout page directly instead of cart page in WordPress this use below code and update it in your Theme Function.php file
add_filter( 'woocommerce_add_to_cart_redirect', function( $url ) {
return wc_get_checkout_url();
});What Woocommerce Does is…
When products are Sold Individually, and when the product already exists in the Cart and customer clicks on Add to Cart, Woocommerce shows Error Message “You cannot add another to your cart…..View Cart”
Instead of the above flow we can change functionality toÂ
When customer clicks on Add to Cart and if the product already exists in the Cart then woocommerce should redirect to Checkout page straightway.
add_filter( 'woocommerce_add_to_cart_sold_individually_quantity', 'vipcomment_change_quantity_to_zero', 10, 5 );
function vipcomment_change_quantity_to_zero( $one, $quantity, $product_id, $variation_id, $cart_item_data ) {
$your_product_id = 3368;
if ( $product_id == $your_product_id ) {
$product_cart_id = WC()->cart->generate_cart_id( $product_id );
$in_cart = WC()->cart->find_product_in_cart( $product_cart_id );
if ( $in_cart ) {
return 0;
} else {
return $quantity;
}
} else {
return $quantity;
}
}
add_filter( 'woocommerce_add_to_cart_sold_individually_found_in_cart', 'vipcomment_is_product_exist_in_cart', 10, 5 );
function vipcomment_is_product_exist_in_cart( $exist, $product_id, $variation_id, $cart_item_data, $cart_id ) {
$your_product_id = 3368;
if ( $product_id == $your_product_id ) {
return false;
} else {
return $exist;
}
}
add_filter( 'woocommerce_add_to_cart_redirect', function( $url ) {
return wc_get_checkout_url();
});