The vast majority of married people are not eligible to to claim the eligible dependant credit. TurboTax doesn't allow it so that people don't accidentally take it. It is a large credit and many people would be upset when the CRA tells them they can't have it, or worse have to pay it back later.
If you believe you are eligible for the credit despite being married, then you can either use TurboTax CD Download and do the necessary overrides, or if you use TurboTax Online, file without the credit, and then send in a paper T1-ADJ T1 Adjustment Request. You may also be contacted by the CRA, and asked for proof that you are eligible for the credit.
