You cannot do a 2017 return using the online version, it's only for 2018. To do a 2017 return, you needed to purchase a desktop/CD version of 2017, and the return could only be filed by mail, since 2017 returns could no longer be e-filed.
Whatever you intended, it appears what you actually did was e-filed a 2018 return using all your 2017 data. The past due obligation is the 2017 return which, as far as the IRS is concerned, you never filed, and still have not.
Now, you need to amend the 2018 return you e-filed, because it was a 2018 return whether you thought so or not, and this time use your 1018 data. Amending a return instructions follow.
You also still need to file a 2017 return, which you could only do by purchasing the 2017 CD/Download version at turbotax.com and sending it to the IRS by printing and mailing it. This is not an amended return, it's an original return because as far as the IRS is concerned, you've never filed a 2017 return. Since the IRS appears ready to take some tax from the incorrect 2018 you filed, you'll want to remember to add whatever they keep and enter it as additional tax you paid for 2017.
If all this sounds pretty complicated, it is, and I would highly recommend you now use a local tax professional who could take care of all this for you.
Amend Instructions:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed#
