How do I get Turbotax to stop treating my Roth 401K contribution as a Roth IRA contribution?

Since your W-2 has your Roth 401(k) contributions in box 12, you do not enter those contributions anywhere else on the tax return.

The problem is that TurboTax is assessing a penalty as if I contributed too much based on the limits for a Roth IRA, not a 401(k)

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@sallydyer57 wrote:

The problem is that TurboTax is assessing a penalty as if I contributed too much based on the limits for a Roth IRA, not a 401(k)

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Back up...Are you also entering Roth IRA contributions in the section for IRA's along with having Roth 401(k) deferrals on the W-2?

What penalty is the program saying you will have?

All I entered was the amount and code (AA)in box 12 from my W-2. The penalty is 6% of the amount above $6000. My understanding is that a Roth 401(k) has a much higher contribution limit.

sallydyer57, somehow you've mistakenly entered this under Deductions & Credits -> Retirement and Investments -> Traditional and Roth IRA Contributions.  Go back and remove the entry you made there.  It is not a Roth IRA contribution.