When you file an extension, it is expected to estimate your tax tax liability and send the amount with your extension. W-2 shows your income and withholdings but it doesn't show anything about tax liability for the year, you can go over your return and turbotax can calculate it for you.
If you do not think you will have any tax liability for the year you can enter 0.
If you do not calculate it correctly, when you file your return you might be subject to late payment penalties but this amount will be very little comparing to late filing without extension. That's why we always recommend people to file extension if they know they need additional time to file their return.
You can also review this article which gives overview if you are subject to late payment penalties.
https://www.irs.gov/taxtopics/tc306
