Why isn't Turbotax adding the educational nonrefundable credit to my federal return? I should be eligible for it as a full time graduate student with high expenses.

[maglib]

Are you claiming yourself?  Did you earn over half your own support from earned income?
If you had completed your undergraduate degree prior to 2018? The only available education credit for you is the Lifetime Learning Credit as you already completed the first 4 years of your undergraduate degree. There is NO refundable portion of the Lifetime Learning Credit.
According to IRS: (2 is the pertinent one)

"Q12.  Who is an eligible student for the American opportunity tax credit?
A. For the American opportunity tax credit, an eligible student is a student who: (1) is enrolled in a program leading toward a degree, certificate or other recognized post-secondary educational credential; (2) has not completed the first four years of post-secondary education as of the beginning of the taxable year; (3) for at least one academic period is carrying at least ½ of the normal full-time work load for the course of study the student is pursuing; and (4) has not been convicted of a felony drug offense."

you can read more here: https://ttlc.intuit.com/questions/1901172-what-education-tax-credits-are-available