Native Instruments Kontakt has two versions, the free player "Kontakt Player" and the full retail version "Kontakt", often referred to as "Kontakt Full".
If you are considering purchasing a library which states Kontakt Full in the Tech Specs 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 open Kontakt you can confirm that it is the full version as it will display Kontakt in the top left. This is 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.