You are correct, this is an unsupported calculation. Since you've already made the maximum permissible elective deferrals at your W-2 employer, you can only make an employer contribution to the solo 401(k). To allow TurboTax to calculate only the maximum permissible employer contribution you must select the Maximize box for either a Profit Sharing Keogh or a SEP contribution. The result will be the same as for an employer-only contribution to the solo 401(k).
