sudo apt update always giving Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)

I had the same issue. It turns out that apt-get uses /etc/apt/apt.conf for the proxy settings (my browser settings are already set). Making apt.conf match my browser proxy settings solved the problem.

/etc/apt$ cat apt.conf

Acquire::http::proxy "http://<proxy>";
Acquire::https::proxy "https://<proxy>";
Acquire::ftp::proxy "ftp://<proxy>";
Acquire::socks::proxy "socks:<proxy>";

Turns out it was a problem in our college's network backend. Issues still exist with it, other WiFi networks work fine.

when you having such issued, first make sure the DNS can resolve the links, which l think is the problem.

sudo vim /etc/resolv.conf

make sure you use google dns :

nameserver 8.8.8.8

as the first name server to check. restart the networking.service and you will be good to go