Don't see anything wrong with both solution, I have changed the function in your code to work with return. Also find the default solution given by SoftUni instructors:

Best,

Code:

function catalogue(arr) {
    let obj = {};

    for (let line of arr) {
        let [productName, price] = line.split(' : ');
        obj[productName] = price;
    }

    let sortedItem = Object.entries(obj)
        .sort((a, b) => a[0].localeCompare(b[0]));

    let string = print2(sortedItem);
    console.log(string);

    // print(sortedItem);

    function print(arr) {
        let lastName = '';
        for (let line of arr) {
            let name = line[0];
            let firstChar = name.charAt(0);

            if (firstChar !== lastName[0]) {
                console.log(`${firstChar}`);
            }
            console.log(`  ${name}: ${line[1]}`);
            lastName = name;
        }
    }

    function print2(arr) {
        let lastName = '';
        let result = '';

        for (let line of arr) {
            let name = line[0];
            let firstChar = name.charAt(0);

            if (firstChar !== lastName[0]) {
                result +=`${firstChar}\n`;
            }

            result += `  ${name}: ${line[1]}\n`;
            lastName = name;
        }

        return result;
    }
}

Instructor solution:

function solve(input) {
    const catalog = {};
    
    for (let line of input) {
        const arr = line.split(' : ');
        const [product, price] = line.split(' : ');
        const letter = product[0];

        if (catalog.hasOwnProperty(letter) === false) {
            catalog[letter] = {};
        }

        catalog[letter][product] = price;
    }

    const sortedKeys = Object.keys(catalog).sort((a, b) => a.localeCompare(b));

    for (let key of sortedKeys) {
        console.log(key);
        const sortedProducts = Object.keys(catalog[key])
            .sort((a, b) => a.localeCompare(b));

        for (let keyP of sortedProducts) {
            console.log(`  ${keyP}: ${catalog[key][keyP]}`);
        }
    }
}