I've checked the server and it appears to be working correctly, which suggests that the problem is perhaps local to you - TerraForm isn't able to connect to the server to verify your subscription and activate itself. It's most likely:
- an internet connection issue.
- a firewall/anti-virus software blocking communications.
Whilst it's not what's causing the problem, the first thing to check is that you have activation keys available. Have you activated before? Is your activation key checked out already? Please see here for info: https://terraformpro.com/docs/installing/
Which anti-virus/firewall software are you using? Where in the world are you based?