You cannot get the child tax credit on a 2020 tax return if you did not have income from working in 2020. In order to get the child tax credit for 2020 you had to have income from working. If you had no 2020 income from working you would not have received any CTC last year.
You can get the child tax credit if you did not work in 2021---but ONLY for a 2021 tax return. But you say you will not be filing for 2021.
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