Your post is confusing but I will take a stab at it ...
1) other parent claimed the child on the 2020 return ... if they were supposed to then there is nothing to be done.
2) their refund was not grabbed for an offset of child support ... maybe they did not have a refund OR their return has not been processed yet OR the offset has not been released yet for distribution (this can take 6 months or more depending on the state)
3) if the child lives with you for more than 1/2 the tax year 2021 then you will claim them on the 2021 return to get the CTC
4) you may be able to get the CTC advance when they open the portal sometime over the next month ...
For more information, see the following links on the IRS web site.
IRS sending letters to more than 36 million families who may qualify for monthly Child Tax Credits; ...
Advance Child Tax Credit Payments in 2021
