The 11.6.0 Unity SDK changed part of the SDK file-structure from previous versions. If you are updating an existing installation to 11.6.0 or newer you will need to delete the following files before you can import the updated SDK:
Once you have deleted these files you can safely import the new unitypackage file. Then go to Window > Tapjoy and make sure that your SDK keys and other settings are still correct. You may also see a warning message about the update – please press the "fix" button by the message to resolve it.