You should be able to enter your W-2 on your individual Federal return in TurboTax as it is shown on your W-2 form, and then make adjustments on Form 8958. Is this what you did last year, and you were able to e-file? Where are you seeing an error message?
According to this IRS FAQ article:
Registered domestic partners should report wages, other income items, and deductions according to the instructions to Form 1040, U.S. Individual Income Tax Return, and related schedules, and Form 8958, Allocation of Tax Amounts Between Certain Individuals in Community Property States.
This older TurboTax tips article on RDP still has some good information.
Regardless of what state you live in, each domestic partner prepares a separate federal return based on the Allocation Worksheet that you attach to it. You might consider including an explanatory note to remind the IRS of your situation.
The creation of a “mock” federal return that reflects joint-filing status is just to get the numbers you need to fill in the sections of your state return that use numbers from a "married filing jointly" federal return. This ”mock” federal return never goes to the IRS; you submit it with your California state return.
See here for California FTB information on married or RDP filing separately.
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