More facts are needed.
Is this your 17-year old stepchild? For IRS purposes, once they are your stepchild by marriage, they are always your stepchild for dependent purposes, even if you get divorced from the other parent.
If this is your stepchild, and they lived in your home for more than half the nights of 2022, you are the only person with the automatic right to claim them as a dependent, and you don't need permission from the other parent. Same for 2023, if they lived in your home for more than half the nights of 2023, you are the only person with the automatic right to claim them as a dependent, and you don't need permission.
If the other parent also claims them, whoever tries to e-file second will be blocked, but they can print their tax return and file by mail. When the IRS gets the duplicate dependent claim, they will send letters to both parents to determine who is really entitled to claim the child as a dependent.
