This may be happening because you have applied for the Canada Workers Benefit (CWB). When you claim the tuition the CWB is not allowed as seen in this TurboTax FAQ: Applying for the Canada Workers Benefit: You do not qualify for the CWB in the following situations:
As per the Canada Revenue Agency (CRA): You have to first claim your current year’s federal tuition fees and any unused tuition, education, and textbook amounts carried forward from previous years on your Income Tax and Benefit Return, even if someone else paid your fees. The amount you must use on your own tax return is equal to the amount of credit required to reduce the taxes you owe.
For more information please see our TurboTax FAQ: How do I claim my tuition fees?
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