If you are referring to a claim for "Eligible Dependant" based on your being a single parent you can make the claim except if any of the following apply;
1) You are required to pay Child Support payments for this dependant, (except in the first year of separation)
2) Someone else is also claiming the dependant for the same credit (you cannot split the credit and if two or more people can make the claim they will need to agree on who will claim the tax credit.)
In order to make the claim in TurboTax you must include the information for your dependant under the "Setting Up" tab and provide information on your marital status.