I am sorry to hear of your loss.
You can't NETFILE a return for someone who has died during the tax year, so your mother's 2020 return would have had to have been sent by mail. You can't ReFile a return that wasn't NETFILEd.
Normally, you would have to use the Canada Revenue Agency (CRA)’s My Account service or send in a request by mail, using a completed T1-ADJ T1 Adjustment Request.
However, as you have received the T5 after the person has died, you can’t just add it to their final return. Instead, you would need to file a T3 Trust Income Tax and Information Return.
TurboTax can’t be used to prepare a T3 return, but Intuit has another program called Profile OnePay that can be used. Profile uses EFILE, not NETFILE to submit returns, so you don’t have an EFILE account, you will need to print and mail your return. It is also for Windows only. You can learn more about Profile OnePay on their website here: Profile Onepay - Single return tax preparation software
This TurboTax FAQ has more information regarding preparing returns for the deceased: Can I prepare a return with TurboTax for someone who has died?
