To qualify for the credit for pension (401(k)) startup costs, the company has must have at least one employee who is not a >5% owner. Since a solo 401(k) likely implies that there are no such employees, it seems likely that this credit would not be permitted. For a sole proprietorship, you can enter the pension startup cost into TurboTax as a business expense, answer TurboTax's related questions, complete the corresponding section under Business Deductions and Credits and, if you qualify, TurboTax will prepare Form 8881.
