If you know you can't file your return by the due date, you don't need to file for an extension. You will automatically be granted an extension until October 15, 2021.
If you anticipate that you will owe additional tax on your return, you should submit your payment with a payment voucher (Form 40V ) with the box "Automatic Extension Payment" checked by April 15, 2021. Any payments made up to May 17 may accrue interest. Any payments made after May 17 may have penalties and interest.