"Contributions cannot be greater than net profit on Schedule C minus deduction for SE taxes minus SE health insurance."
Yes, that's correct. That's what the tax code says and TurboTax adheres to that. The sum of the deductible portion of self-employment taxes, your self-employed health insurance deduction and your contributions to the solo 401(k) are not permitted to exceed the net profit shown on Schedule C line 31. You'll need to limit the amount that you claim as a self-employed health insurance deduction or reduce your solo 401(k) contribution so the sum of these does not exceed net profit from self-employment.