I watched a Kelby One lesson and per course purchased a Yongnuo YN560-TX manual controller to trigger my Nikon SB900 off camera with a softbox. Do I need to purchase something else to make this work (R-603 II or something). New to this and didn't mention in class. I have searched everywhere trying to figure this out because it doesn't work as is with D600, SB900, and the YN560-TX II manual controller.
The YN-560-TX, as a dedicated radio transmitter requires a radio receiver to trigger a remote flash. The SB-900 does not have one built-in, so you need to add one onto the foot of the flash. You can use the RF-602, RF-603, RF-603 II, or RF-605. But all you will be able to do is fire the flash remotely. The YN-560-TX and an RF-60x cannot communicate TTL, HSS (use a faster shutter speed than your camera body's sync speed), or remote power/settings control. This gear is manual-only.
You would actually have more control (TTL, remote settings control) over an SB-900 if you used it as a "smart" optical (CLS/AWL) off-camera flash while using the D600's pop-up flash as your CLS commander.
To get similar control over radio, you could also use iTTL-capable radio triggers: the Yongnuo YN-622N, Godox X1R-N, Phottix Odin II, etc. etc. There are dozens of these types of radio triggers on the market today.
The YN-560-TX is actually best paired with the YN-560 III, YN-560IV, YN-660, or YN-560Li units, which have built-in radio triggers that allow for remote zoom and power control. But. In Yongnuo's system, you cannot mix manual and TTL gear, as you can in Godox or Phottix's systems.
The Yongnuo YN560-TX II has a radio transmitter made to communicate with Yongnuo manual flashes with built in radio receivers that use the YN560/RF605/RF603 protocol.
Your SB900 has an optical receiver for Nikon's CLS/AWL wireless flash protocol.
Although the Yongnuo YN560/RF605/RF603 manual protocol and the YN622 TTL protocols are mostly incompatible, YN622 devices made since the end of 2014 can be set to receive YN560-TX signals to control manual flash power and manual zoom head settings as well as fire the flash. If you set a YN622N II to 'YN560 Rx' mode and attach it to your SB900 it should allow you to control the power and zoom head setting manually from the YN560N-TX attached to your camera and fire the flash in sync.