Foster children can be viewed as the same as biological children for tax purposes as long as the child was placed with you by an authorized placement agency or by Judgment, decree or other court order. Obviously there are tax benefits to being able to claim your foster child as a dependent...such the Child Tax Credit and the Earned Income Tax Credit.
As for your 2021 tax return, it most likely will be beneficial for you to amend if the child lived in your home more than 1/2 of the year (183 Days).
IRS Publication 503 is a helpful resource:
https://www.irs.gov/publications/p503
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