It seems like some of the Indian APT repository servers for Ubuntu are either temporarily down or have a major issue. I would exclude a problem with your DNS for now, if you are able to reach normal Internet, at all.
Comment out the old entries in your /etc/apt/sources.list and replace them with something that definitely works.
- Comment out the current entries.
sudo nano /etc/apt/sources.list
-
Search on https://duckduckgo.com/ for Ubuntu repository servers. There should be a website somewhere that shows you a complete list of all available Ubuntu repositories.
-
Select a link from the list and
pingit. If successful, visit the website and see if the repository directories are walkable. If again successful, add this link to your/etc/apt/sources.listand executesudo apt update.
If at least one of the previous conditions was unsuccessful, try another link from the list and repeat the process. -
If
apt updatewas successful in the last step, you should be able to installtasksel, now.