Why can’t my 17 year old get the earn credit if she was 16 in 2023

Are you referring to earned income credit which dependents can’t receive or some other credit?

a taxpayer who can be claimed as a qualifying child, even if not claimed, doesn't get the earned income credit.

you can claim the person as a qualifying child if all these tests are met:

1) Related to you - son. daughter. stepchild, eligible foster child, or a descendent of any of them; brother, sister, half-brother, half-sister, stepbrother, stepsister, or a descendent of any of them. Adopted child if legally adopted or an individual lawfully placed with the taxpayer for legal adoption. An eligible foster child is one placed with the taxpayer by an authorized placement agency or by judgment decree or order of any court with jurisdiction
2) Same principal abode as you for more than ½ the tax year.
3) Under 19 at the end of the tax year.
4) Hasn’t provided over ½ their own support
5) Hasn’t filed a joint return – unless there is no tax liability
6) A US citizen, resident alien or national, or a resident of Canada or Mexico

When did she turn 17?  Had to be 16 or under on Dec 31, 2023