This can happen when you still need to enter some details from the student's return into your return in TurboTax to complete the calculations.
There are two places that will need details from the student's return:
1) Deductions > Additional Dependant Information. This is where you enter the student's net income from line 236 of their T1, and the total of their non-refundable credits from lines 3 to 18 of their Schedule 1.
2) Deductions > Claims for Dependants > Claims for Dependants Intro, select Yes, click Continue, click Enter Credit Info, check off Transfer my child's tuition and education amounts, and click continue. Make sure you complete all of the applicable fields on this page including unused federal and provincial tuition carried forward from 2017, deductions from net income from line 257 of their T1, and provincial non-refundable credits (total from lines 5812 to 5840).
If the student also qualifies for the disability tax credit, you will need to visit that page as well and select Yes for those credits to be included in the calculation of available tuition amount for transfer.