American Opportunity Credit

 The usual reason is the his scholarships exceeded his qualified expenses.  Schools sometimes mistakenly include loan money in the scholarship box 5, on the form 1098-T. It's easy to make mistakes in the TurboTax education interview.  If a  529 plan distribution is involved, TT may have  failed to divert some of the expenses to the AOC credit

It is not too late to file amended returns for 2018-2020.  The deadline for amending 2018 is 4-15-22.  

The 1098-T is only an informational document. The numbers on it are not required to be entered onto your tax return. You claim the tuition credit, or report scholarship income, based on your own financial records, not the 1098-T. 

You only need $4000 of qualifying expenses to claim the maximum credit ($2500, $1000 of which is refundable). There's even a  “loop hole” available, when the student has scholarship. The student reports all his scholarship, up to the amount needed to claim the American Opportunity Credit (AOC), as income on his return. That way, the parents  (or himself, if he is not a dependent) can claim the tuition credit on their return. They can do this because that much tuition was no longer paid by "tax free" scholarship.  You cannot do this if the school’s billing statement specifically shows the scholarships being applied to tuition or if the conditions of the grant are that it be used to pay for qualified expenses.

Using an example: Student has $10,000 in box 5 of the 1098-T and $8000 in box 1. At first glance he/she has $2000 of taxable income and nobody can claim the American opportunity credit. But if she reports $6000 as income on her return, the parents can claim $4000 of qualified expenses on their return.

Books and computers are also qualifying expenses for the AOC. So, extending the example, the student had another $1000 in expenses for those course materials, paid out of pocket, she would only need to report $5000 of taxable scholarship income, instead of $6000.