It's working now. There are just a couple workarounds. Try in a different browser from the one you are using; usually Chrome works. If not, try Firefox. You may need to also clear browser cache/cookies and then try steps below.
The file an extension link under "Manage your returns" is not working properly and still being investigated by the developers right now.
So when you log into your TurboTax account, click Tax Home to the left (do not click Take me to my return) and then look in the bottom left corner above Sign Out and click File an Extension there instead.
Or try going here <a rel="nofollow" target="_blank" href="
https://turbotax.intuit.com/irs-tax-extensions/">https://turbotax.intuit.com/irs-tax-extensions/</a> and click File a Free Extension and use your TurboTax information to log in and go through steps from there.
