turbotax premium. I need to enter a solo 401k contribution. but I don't see 'other business expense' section or "self-employed retirement' section. where can I enter it?

Using TurboTax Online Premium, go to the Wages and Income section, then scroll to the bottom until you see Other Business Situations.  Expand that section and then you will find Self-employment Retirement Plans where you can click Start to enter your information.