Cannot Adopt an already existing Sim

There's two different interactions:

1. "Adopt as caregiver" - this does not change the actual family relationships of the Sims. It just makes the adult Sim responsible for the infant/toddler so that they queue check infant/toddler interactions and autonomously care for them. I don't believe it's available past toddler, as you no longer need an adult to take care of basic needs like feeding, bathing, and putting to bed. "Adopt as caregiver" is base game.

2. "Adopt" - this does change the family relationship of the Sims, but is only available in specific circumstances. First, I believe it's part of Growing Together, not base game. And second, it only works if the child doesn't already have 2 parents in their family tree. It doesn't matter if one or both parents are dead, or even if they've never met them - if the parent slots are filled, you can't add more parents.

Technically there's a third action - "adopt" though phone or computer to adopt a new randomly generated child - but that's not an interaction available between existing Sims.