Recently announced in October, 2017 by the US government, is that all visitor visas by US citizens to Turkey have been suspended. Then in November, the decision was reversed. US Citizens can now apply for a pre-arrival visa but can not use the e-visa process upon arrival. Make sure you leave ample time for your visa application to be processed as appointments are required.. You may be able to avoid this by entering from another country and not from the US.
Since it is now in a state of flux, make sure you check for recent developments before you book your trip.