There are 2 possible reasons that I know of:

1. Another user on my forum (Movie Studio Zen Forum), had a similar problem many months ago.
After a very long investigation by myself and Magix support, the problem was discovered.

Have you ever installed any Pirated "cracked" versions of Vegas on your computer ?

If you have, this is what is causing your problem.
There is a black list of email addresses used by software pirates. These black listed email addresses are used to register pirated versions of Vegas and are known by Magix. Pirated versions of Vegas, screw up the activation system for Vegas. It does something to your computer's Windows Registry and makes it impossible to install official Magix versions of Vegas. Simply un-installing the pirated Vegas software will not fix the problem, because the pirated "cracked" software has damaged Magix software installations.

Magix Software cannot help you fix this problem, since they have nothing to do with pirated software.
If this is what you have done, the only solution is to do a Clean Install of Windows, which will wipe all remains of the pirated software from your computer. You will then have to re-install all your Drivers and Programs.

2. If you have never installed any pirated software, you may have exceeded your allowed amount of activations for Vegas Movie Studio 14 Suite. Login to your Magix account and find Vegas Movie Studio 14 Suite. Check to see how many activations have been made. 2x is the maximum. If 2x activations are showing, deactivate both of them. Take special note of the email address you use for your Magix Account and make sure to use the same email address when registering Vegas.

Regards
Derek