No, if you defer all your self-employment income into a Solo 401(k), you would not show any taxable income for the year. Without any earned income, you cannot contribute to a Roth IRA.
According to the IRS, your contribution limit is the lesser of:
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$6,000 ($7,000 if you are age 50 or older) minus all contributions (other than employer contributions under a SEP or SIMPLE IRA plan) for the year to all IRAs other than Roth IRAs, or
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Your taxable compensation
