The Marketplace says my Daughter is Eligible for Medicaid/CHIP, can I add her to My Plan?

Since she may be eligible for Medicaid/CHIP, it is likely not in your financial best interests to enroll her in a Marketplace QHP because you will not be eligible to receive either APTC or CSRs and would be required to pay the full premiums for her coverage. If you insist on having your daughter enrolled in an individual plan, you should submit a new application either directly to the carrier (off-exchange) or submit a new application through healthcare.gov and indicate you do not want help paying for coverage 

You can appeal her eligibility status, but the appeals process is for consumers who disagree with the eligibility determination, so it would be inappropriate (and a waste of time) to file an appeal in this situation.