From the VEGAS Support help pages, error 14 advice is:

You successfully installed your software, but you still receive the message "Error code -14" when you try to register it.

To solve your problem, first check the indicated serial number for typos. In case you purchased several applications at the same time, please check if you were using the serial number matching to the program.

To confirm the serial number being used, go to the Vegas Pro 15 listing in My Products on your MAGIX My Account where you'll find VP15's serial number.

Also, make sure that you are using the email address that you used at the time of registering/activating Vegas Pro 15 originally.