I don't know. I had mentioned to the agent that I was considering claiming CTC for 2018 or earlier, and that's when the agent told me that in order to do that I had to make adjustment requests for previous years. But if I were to guess, then I would assume that you can claim the most recent of the two options below:
2019, since that's the first year the CTC was offered, OR
As far back as you are allowed to make adjustment requests.
But again, those are guesses. I can't give any additional guidance as I'm not going to be making adjustment requests.