Contributions to the Solo 401(k) are not permitted to exceed your net earnings from self-employment. Net earnings are net profit minus the deductible portion of self-employment taxes. With $7,000 of net profit, your net earnings are $6,505 (assuming that you do not also have W-2 income that would result in your total compensation exceeding the Social Security wage base). That means that the maximum employee Roth contribution to the Solo 401(k) would be $6,505.
TurboTax is incapable of flagging an excess contribution to the Solo 401(k) that is excess only because the contribution exceeds net earnings (the section 415(c) per-plan limit) . It only checks the entry against the elective-deferral limit (the section 402(g) per-individual limit)
