Main issue was only in Reset command!

;-)

function plantDisc(input) {
    let nLines = Number(input.shift());
    let plantObj = {};

    for (let i = 0; i < nLines; i++) {
        let [plant, rarity] = input.shift().split(`<->`)
        plantObj[plant] = {
            plant,
            rarity,
            rating: 0,
            count: 0,
        };
    }

    let currentRow = input.shift();

    while (currentRow !== `Exhibition`) {
        let command = currentRow.split(` - `).join(` `).split(` `);

        if (plantObj.hasOwnProperty(command[1])) {
            switch (command[0]) {
                case `Rate:`:
                    let counter = 1;
                    let plant = command[1];
                    let rating = Number(command[2]);

                    plantObj[plant][`rating`] += rating;
                    plantObj[plant][`count`] += counter;

                    break;
                case `Update:`:
                    let plantT = command[1];
                    let newRarity = Number(command[2]);
                    plantObj[plantT][`rarity`] = newRarity;

                    break;
                case `Reset:`:
                    let resetPlant = command[1];
                    plantObj[resetPlant][`rating`] = 0;
                    plantObj[resetPlant][`count`] = 0;
                    break;
            }
        } else {
            console.log(`error`);
        }

        currentRow = input.shift();
    }

    console.log(`Plants for the exhibition:`);

    for (const key in plantObj) {
        let averageCount = plantObj[key][`rating`] / plantObj[key][`count`] || 0; // if falsy => 0/0, then return 0
        console.log(`- ${key}; Rarity: ${plantObj[key][`rarity`]}; Rating: ${(averageCount).toFixed(2)}`);
    }
}