Please check to be sure the YN622N-TX's hot foot is properly seated all the way in the camera's hot shoe. Sometimes new flashes or controllers feel like they are all the way in before they actually are. This is particularly the case if the hot shoe has not been used before. There have also been reports that some of the newer Nikon camera models (particularly the D750)¹ have slightly modified the dimension of their hot shoes and more care must be taken to properly seat the flash or controller over the contacts.

I have a YN-622N-TX on top of my D750 and it works fine(¹)!!!

(¹) Nikon made a slight change to the flash shoe on the D750. It is a bit deeper, so pins don't align quite as easily, however, this is simple to get around. Mount the yn622 on the camera, then lightly tighten the locking wheel. This will extend the alignment pin on the bottom of the yn622. GENTLY pull back on the unit until you hear and/or feel the pin pop into place. Lock it down the rest of the way now. - Rick Moncello

If that isn't the issue, then it's probably this:

The flash is set in optical "slave" mode and the contacts in the hot foot are turned off. The flash is monitoring the optical receiver for a pulse of light that tells it to "fire." Thus the flash is not detecting the presence of the YN622N receiver on the flash's hot foot. Take the flash out of slave mode so that it can be controlled by the receiver attached to the foot of the flash.

¹ Please see the following links for more:
The comment by Rick Moncello left September 10, 2015 at the amazon review section for this set of two Yongnuo YN622N transceivers .
This Nikon Rumors post: Phottix: the Nikon D750 hot shoe has different dimensions than previous camera models
This Nikonites thread, which also mentions the D5500.