Unable to install Lightroom 6

Hello again,

I want to install Lightroom 6 (DVD Version) on a Macbook with macOS High Sierra. When I install and try to register the Software with the Serial Number, I always get an Error Message "There is no Connection to the Internet or time is set wrong on the computer". But I have a Connection. I also tried in different Places. Time and Date is also set correct.

So after seven days of free trial, I cant open Lightroom anymore.

So I did a full reset of the Macbook and installed it again. Same Problem. I dont know what to do anymore? Do I need to install other additional software?

I hope anyone can help me. 

Thank You.