Hello,
Since your capital losses were in 2020 and not in 2019, they should be filed in your 2020 tax return.
I believe you will have to file Form T1A to request for Loss Carryback when you file your 2020 return.
More information can be found on the CRA website:
https://www.canada.ca/en/revenue-agency/services/tax/individuals/topics/about-your-tax-return/tax-re...
The site above also mentions to not file an amended income tax and benefit return for the year to which you want to apply the loss. Therefore, you shouldn't amend 2019 with 2020 losses.
Sincerely,
