To be able to contribute at all for 2016, you were required to have established the one-participant 401(k) plan before the end of 2016. If you were not aware of the option prior to August 2017, it seems likely that you had established such a plan by the deadline.
As SweetieJean said, to be able to make an elective deferral for 2016 you were required to have made the deferral election by the end of 2016. Even if you didn't make an elective deferral election, if you had established the plan by the end of 2016 you can make a profit-sharing contribution up to the due date of your 2016 tax return, including extensions.
