Form 8978, Partner’s Additional Reporting Year Tax, is not supported by TurboTax.
But according to the Instructions for Form 8978, the form must be submitted with your return. This means you will need to print your entire return and file by mail, including completed Form 8978 and Schedule A, if applicable.
Note for TurboTax Desktop: The adjustment from Form 8978 is entered on Schedule 3 Line 1L, but you cannot make a manual entry to this form in TurboTax. The total from Schedule 3 is shown on Form 1040 Line 20, where you can use override to enter an amount. Note this is a non-refundable credit.
Download Form 8978 and instructions here: About Form 8978, Partner’s Additional Reporting Year Tax