This is somewhat of a known issue with respect to Form 8960 and the online versions of TurboTax.
I suggest you contact Support (see link below) as you will have to use an installed (desktop) version of TurboTax to circumvent the error.
https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632
