When your business expenses exceed your business income and you incur a "Non-Capital Loss", you may not be able to use it to offset other personal income this year.
Just because you can't use the loss this year to reduce your taxable income, doesn't mean that you won't get to use it at all. Non-capital losses can be used to offset other personal income in any given tax year and can be carried back three years or carried forward for up to 20 years.
The loss itself is calculated in the software when you complete the T2125 - Business Activities Schedule.
To carry a non-capital loss back to 2014, 2015, or 2016 in TurboTax, you will be asked about this in the Review section. When asked about applying your losses in the Tax Savings Opportunities section, click "Explore" > "Applies to Me", then complete the information on the screen that appears (example in the screenshot).
If you are not applying your 2017 loss to any of the three previous years, TurboTax will carry forward the unused amount for you and it will show on your CRA 2017 Notice of Assessment.