Jaskirat,
Sorry that you are seeing this installation issue.
Are you able to install any other plugins from the Marketplace? If you cannot open the marketplace, then there seems to be a network issue.
Please see this FAQ on how to configure proxy settings in Eclipse IDE : https://www.genuitec.com/forums/topic/offline-installation/
If you are not behind a proxy or firewall but still seeing issues accessing the marketplace, then please switch to a new workspace, try opening the Marketplace and share with us the .log file located at workspace dir/.metadata/.log (rename the extension to .txt and attach it here) to help us investigate further.
Alternatively, you can install DevStyle in offline mode. Please see this post for the detailed instructions : https://www.genuitec.com/forums/topic/offline-installation/
Hope this helps. Do let us know how you get on with the installation.
–Swapna
Genuitec Support