Why i can't register 2.6.8 on my mac ?
That could be one of beta versions that we had. Can't say for sure.
But that version you should skip anyway because it suffered a ton of changes under the hood (and 3.7 as well)
Register? Or install?
I think Traktor Pro 2 is no longer supported on latest mac. Might be wrong though. Better wait for someone else to confirm it.