To get to Schedule C, try going to Wages and Income > Self-Employment Income and Expenses if you are using TurboTax Online.
Try going to Wages and Income > Business Items > Business Income and Expenses (Sch C) if you are using a desktop version of TurboTax except for the Home and Business version. If using Home and Business, there is a Business tab at the top of the screen and Schedule C is found after clicking that tab.
Once you have gotten to the Schedule C inputs, be sure to enter the income document that was trying to link to the Schedule C if it does not already appear in the income section.