No, there is not an adoption credit for receiving legal guardianship, only if you actually adopted the child, then this credit is based on fees you paid.
However, if the 17-year-old was placed with you by an authorized placement agency, judgement, decree or other order of any court of competent jurisdiction then you can claim him or her as a dependent for the Other Dependent Credit and treat them as your qualifying child even if they didn't live with you for all 12 months of the year.
https://taxmap.irs.gov/taxmap/pub17/p17-016.htm#TXMP49a43bd5
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