Thank you for reporting this issue. The inspector fails because it cannot find any .wtm files anywhere in the StreamingAssets folder. If you want to tick 3D Tracking in the editor, you should have at least one .wtm file recorded in advance and placed in StreamingAssets. After that, the check will work properly and you will be able to select the .wtm file in the Target Collection dropdown. The error will be fixed in the next release.
about 2 years ago
Seems like the ClientTracker.dll script won't allow you to tick the checkmark box for '3D Tracking' in the inspector - it'll give the error "Array index is out of range". It does, however, work only if you select 'Use custom URL' for the target collection, at which time you cannot uncheck 'Use custom URL' after checking 3D tracking (quite the mouthful)
To test yourself, open the 'Simple' Client Recognition sample scene and tick '3D Tracking' on the Client Tracker script.