If you paid it in December 2018, it should be included in Box 1 of the 2018 1098-T. If it isn't, you can adjust for that, but you need to have proof that the payment was made in 2018. Compare the 1098-T to the student's school account statement.
You can use your student account statement and add payments that posted to the school if they are not properly posted in Box 1. Be sure you keep a copy of the account activity to prove the payments were made in 2018 with your tax file.
In TurboTax, enter the 1098-T and click the blue "What if this is not what I paid?" link below Box 1 on the 1098-T screen. When the next screen opens, you may need to scroll down just a bit to enter the amount that was paid to the school.
Please see screenshots below
