I need to file an extension for my 2018 taxes BUT it is asking me how much I owe back in taxes for last year.

[BurakG]

Filing an extension will save you from "Failure to file" penalties but you are still required to pay amount you owe (If any). When you go thru the interview screen turbotax will ask you all the questions and will give you an estimate that you can enter. 

If you think you are not going to owe anything you can enter 0 there. 

You can find more information about penalties and extension here 
https://turbotax.intuit.com/tax-tips/irs-tax-return/filing-your-taxes-late/L7IhvwH9b