Hmm, not sure if reduce is the right way to go here, better perhaps to first practise Array methods in combination with if/else validations.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/at

function solve(strings) {
    let items = strings.shift().split(', ');

    let result = strings.reduce((a, v) => {
        const [command, itemList] = v.split(' - ');

        if (command === "Collect" && !a.includes(itemList)) {
            a.push(itemList);
        } else if (command === "Drop" && a.includes(itemList)) {
            a.splice(a.findIndex(x => x === itemList), 1);
        } else if (command === "Combine Items") {
            const [oldItem, newItem] = itemList.split(':');
            const oldItemIndex = a.findIndex((i) => i === oldItem);

            if (oldItemIndex !== -1) {
                a.splice(oldItemIndex + 1, 0, newItem);
            }
        } else if (command === "Renew" && a.includes(itemList)) {
            const itemIndex = a.findIndex((i) => i === itemList);

            if (itemIndex !== -1) {
                a.splice(itemIndex, 1);
                a.push(itemList);
            }
        } else if (command === "Craft!") {
            return a;
        }

        return a;
    }, [...items]);

    console.log(result.join(', '))
}