$product_id
has no value in your code
function cw_change_product_price_display( $price, $product ) {
$sell_price = get_field('sell_price', $product->ID);
$sell_duration = get_field('duration', $product->ID);
$price .= " | SELL ₦$sell_price in $sell_duration";
return $price;
}
add_filter( 'woocommerce_get_price_html', 'cw_change_product_price_display' 10, 2);
add_filter( 'woocommerce_cart_item_price', 'cw_change_product_price_display' 10, 2);