You have not really given us a lot of information, unfortunately! e.g., what error message, if any, do you see?
It is possible you have exceeded the maximum number of simultaneous activations which the software allows. If that is the case, go to your Magix account, log in, go to My products and check the information listed for your software. If it includes activations that are no longer needed, select to deactivate them.
Then try again to activate your new installation.
If this does not work you will need to contact infoservice@magix.net and ask for their help.