I bought a brand new traktor z1 from amazon.com and when I went to register the software to download the traktor pro 4, there was a message that the serial number was already registered.
I have no idea what to do now.
Contact NI support with a proof of purchase.
https://support.native-instruments.com/hc/en-us/requests/new
Or return your Z1 to Amazon as it isn‘t brand new obviously. (Maybe this was returned by someone.)
You have to contact support:
https://support.native-instruments.com/hc/en-us/articles/360014498698-How-to-contact-NI-Support
Thanks for the advice! I contacted Amazon and will return the product.