Why does it show I owe taxes to Ontario province when my employer already paid for it? I can't edit the line in TurboTax

You can't edit that line because it's automatically calculated, based on your whole return, including your t-slips. 

There are a couple of reasons why you might have a balance owing.

Your employer may not have taken enough taxes off your pay.

You have other other income that you being taxed on (bank interest, capital gains etc...)