A solo 401K is not a deduction on the Sch C so there is nowhere to enter it. Only contributions by the employer to employee accounts are a deduction.
To enter your self-employed retirement contributions in TurboTax, log into your tax return (for TurboTax Online sign-in, click Here) and type "self-employed retirement" in the search bar then select "jump to self-employed retirement". TurboTax will guide you on entering this information.
OR use the tax topics tool ... under the tax tools >> tools >> tax topics
