After purchasing KONTAKT 6 or KOMPLETE 12 (ULTIMATE), Native Access only shows Kontakt in the Not installed tab.
The successor of KONTAKT 5 is only called KONTAKT in Native Access. The same applies to the name of the application and the plug-in version.
Just go ahead and install KONTAKT. You can find the current KONTAKT version number on the right side of the Kontakt entry in Native Access.
Note: KONTAKT 6 is compatible with macOS 10.12 and higher. If you are using an older operating system, KONTAKT 6 will not appear in Native Access.