When I enter my tuition payments for 2018 my tax refund gets reduced significantly. Why is that?

You are not eligible for the working income tax benefit if you were enrolled full time as a student for more than 13 weeks in the year. Prior to entering your tuition fees, TurboTax may have calculated the working income tax benefit for you which would increase your refund. Once you entered your tuition fees, this claim would have been removed, therefore, reducing the amount of your refund. For more information, you may want to contact the Canada Revenue Agency at 1 800 959 8281.