Turkey is taking delivery of the Russian S-400 defence system but still expects to get F 35 planes from the US. Turkey is a member of NATO with a pretty big army.
The US will not allow any country to operate both F-35 and a top of the range Russian defence radar/missile system.
The reason is simple. In peacetime western stealth aircraft normally fly with radar reflectors fitted (typically Luneberg lenses). There are at least two reasons for this - firstly it means they can be seen by ordinary (civil) radar systems so they don't accidentally 'bump' into other traffic, and secondly to disguise their true radar signature from prying non friendly radars. Israel, the US and the UK have all operated F-35 close to 'hostile' countries such as Russia, Syria and Iran.
The only time the radar reflectors are removed is when the planes are taking part in active (potentially hostile) missions. There is still a chance that a hostile radar may pick something up, but given the element of surprise, and the lack of information on what a 'cloaked' stealth aircrafts radar signature looks like chances are the stealth aircraft will complete it's mission safely. The SS-400 operators in Russia, Syria and Iran won't get any practice in targeting a cloaked F-35, and if push comes to shove an F-35 will have taken out all the SS-400 sites before the SS-400's operators have learned what to look for.
However, if Turkey is allowed to operate both F-35 and SS-400, then they can work out how to use the SS-400 to target the F-35 in peacetime conditions, and spend as long as they want doing so. And the risk is that the information on how to do it will get fed back to Russia, and then on to Iran and Syria. The US cannot allow that to happen, since it'll remove one of the major advantages the F-35 has, and F-35 is key to western defence for the next 20+ years.
Turkish pilots have already been banned from areas of Luke AFB in Arizona (the training base for F-35A's), and their aircraft impounded. None have been delivered to Turkey so far. And the way things are going, none ever will be. The US won't want to loose access to Incirlik AB in Turkey, so some tough decisions will be needed soon.