As per CRA- 

Your repayment period starts the second year after the year when you first withdrew funds from your RRSP(s) for the HBP. For example, if you withdrew funds in 2019, your first year of repayment will be 2021.

Note 

If you choose to start your repayments earlier, your repayment period will stay the same. Any repayments made before you are required to start your repayments will reduce the amount you have to repay for the first year.