It is preferable to file your 2020 tax return before your 2021 tax return. Some items on your 2020 return may affect your 2021 return, so I would recommend doing them in order if you can. For example: if you paid tuition in 2020, in order to carry forward any unused amounts to use in your 2021 return they would first need to be recorded on your 2020 return. If you filed your 2021 returnfirst, you would not be able to use these amounts yet, as they have not yet been recorded in the year they occurred.
"We recommend that you file the oldest return first to take advantage of carry-over amounts that may be available from the previous year. The CRA will charge you a late-filing penalty if you file your 2021 tax return after the due date and you owe tax that remains unpaid at that time."
As well, if you file your tax return after the due date and have a balance owing, you will be charged a late-filing penalty. Filing late may also cause delays to your benefit and credit payments.
For more information please refer to the following CRA article: