If you are using the Forms mode of TurboTax, both lines 128 and 156 are fillable lines.
If you're unable to directly place your cursor in either on these lines, ensure that your copy of TurboTax is up-to-date.
Alternatively, you can switch to the Easy Step Interview and enter the amounts. Use the Find button to search/select the option for Support Payments Received.
https://turbotax.community.intuit.ca/replies/2647741
