Error was at the cityPrice function:

result = `${key} -> ${obj[key]}`

should be:
 
result += `${key} -> ${obj[key]}`

Otherwise good code !

100%:

function solve(input) {

    let destination = generateDestination(input);

    function generateDestination(arr) {

        let obj = {}

        arr.forEach(line => {
            let tokens = line.split(" > ");
            let [country, city, price] = tokens;

            if (!obj.hasOwnProperty(country)) {
                obj[country] = {}
            }

            if (!obj[country].hasOwnProperty(city)) {

                obj[country][city] = price;
            }

            let oldPrice = obj[country][city];

            if (oldPrice > price) {
                obj[country][city] = price;
            }

        });
        return obj

    }

    let sorted = Object.keys(destination).sort((a, b) => a.localeCompare(b));

    for (const country of sorted) {
        console.log(`${country} -> ${cityPrice(destination[country])}`);
    }

    function cityPrice(obj) {
        let result = [];
        for (const key of Object.keys(obj)) {
            // result += `${key} -> ${obj[key]}`
            result.push(`${key} -> ${obj[key]}`);
        }
        return result.join(' ');
    }
}