YES you can. All you need is your 2018 Military W-2 and indicate that you were in the military in 2018. Where you are now is immaterial.
But Free and Deluxe Online are only free to use and file for E-5 and below. IF some of the tax forms needed end up having to use the Premier or SE editions, then those won't be free......and that can happen for some people's situations. (Be careful that you do NOT upgrade to the LIVE versions either...none of those are Free for Military)
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*