I bought the upgrade from traktor 2 to 4 for 78$ but when I write the serial it doesn't register it correctly. Also it seems that by mistake I bought two traktor, but that doesn't matter now. What should I do?
What do you mean by "it doesn't register it correctly"? Do you get an error?
You should probably contact support:
https://support.native-instruments.com/hc/en-us/articles/360014498698-How-to-contact-NI-Support