Native Instruments Kontakt comes in two versions, the free player which is called Kontakt Player and the full retail version which is simply called Kontakt but also often referred to as "Kontakt Full" to help distinguish it from Kontakt Player.
If you are considering purchasing a library which states Kontakt Full Required in the key stats on its product page, you will not be able to load it in the free Kontakt Player. To make sure you have the required version installed, you can try one of the following steps:
A. Open the plugin/app and check
If you simply open Kontakt (either as a plugin or as a standalone application) you can quickly confirm that it is the full version as it will display Kontakt in the top left (as outlined in green below). If you only have Kontakt Player then this will display the words Kontakt Player instead.
B. Check Native Access
The other way to check is to look in your Installed Products list in Native Access, this will display either one of the following options. 
Please note, if Kontakt Player is in the list instead it will also have the text Full Version on the right hand side, this does not mean it is Kontakt Full.