There are two steps to fix this issue:
1. Check the primary IP of your domain here - http://verify.cpanel.net/index.cgi . It will show whether or not cPanel/WHM license is active for your IP. If you find that the license is active, go to step 2, otherwise, contact VPS provider or the company who gave you cPanel/WHM account.
2. Login to your VPS as root and run this command: /usr/local/cpanel/cpkeyclt
Step 2 should fix the issue if you get the following message after running the command-
Updating cPanel license...Done. Update succeeded.
Building global cache for cpanel...Done