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.
Did you find where to enter the solo 401K ? Where to enter Self Employed solo 401K plans
You can contribute to a 401K up to the amount of your Net Profit reduced by 1/2 of the SE tax OR to the max contribution allowed for a 401k whichever is Less.
You might qualify for the Savers Credit
https://turbotax.intuit.com/tax-tips/tax-deductions-and-credits/what-is-the-savers-credit/L3LyopRkK
I don’t know where it is. It might be under
Federal Taxes Tab
Deductions & Credits
-Scroll down to:
--Other deductions & Credits
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.