With error 72, there seems to be only 2 instances on the forum relating to this error code but with no specific solution.
Try uninstalling and re-installing Vegas Pro again. If that doesn't work, you'll need to contact Support via the Support button at the top of the forum. It's a convoluted process but persist with it and you'll eventually get to the Send a Support Request button.
If you need to send a support request, make sure to identify which Vegas Pro version and build you are attempting to activate along with its serial number.