Why is it not allowing me to enter my self employed business expenses?

Which version of TurboTax are you using?  Online or Desktop?

In general, you are asked for the expenses after entering the income. 
 

To get there you will select the following:

1. Business income and expenses
2. I'll choose what I work on
3. Start or update next to Business income and Expenses
4. Edit next to your business profile
5. Start next to Business Expenses