You will be paying tax on it in the year that you actually receive the money. If you receive the money in 2020 it will be taxable on your 2020 tax return. If they do not pay it to you until next year in 2021, then it will be on your 2021 tax return--which you will file in 2022.
But talk to your employer--they should certainly be able to explain to you when you will receive the bonus--and they will have to withhold tax from the bonus. It will be on your W-2 for the year that they pay it.