Because the Solo 401(k) was not set up before 2022 and you therefore could not by the December 31, 2021 deadline make the election to make an employee contribution, you are not permitted to make an employee contribution for 2021. You can only make the employer contribution which is a maximum of 20% of net earnings. Net earnings are net profit from self-employment minus the deductible portion of self-employment taxes.
To get TurboTax to calculate the maximum permissible employer contribution, mark the Maximize box for a SEP contribution, not a 401(k) contribution. Marking the Maximize box for a 401(k) contribution would cause TurboTax to assume that you are eligible to make an employee contribution, which is not true in this case.
