You can’t claim any tax credits if tuition was already paid with tax-free money.
Now, if you had paid room and board, then you could consider part of that is tuition eligible for the credit, in this way:
Suppose $10,000 of tuition is paid from the 529 and you paid $5000 room and board out of pocket. You could treat the 529 payment as going $5000 to tuition and $5000 to room and board, and treat the $5000 you paid as going toward tuition, and then claim the credit.
But since you paid nothing, then nothing is eligible for the credit.
