"Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)" This error message usually indicates one of the following: a misconfigured proxy server, a misconfigured network server, a misconfigured VM bridge to the network connection on a Windows machine running a Linux VM. The bionic msql package is also a problem. Just a guess here in your case, your proxy or network server is using older code than your system. Try a different mirror first just to make sure that your mirror is up. I checked your link and the files are unsigned. I know how to fix this in Debian, but I better go check a Linux Lite system first. I'll add to this shortly.
OK. First a few questions. Is your Linux Lite system running as a VM, and if so is the VM host Windows or Linux? If not, are you connecting through a proxy server, or through an institutional network like a library, business, or school, or through a cell hotspot, in order to connect to the internet? In the first case you have to allow secure connections and http connections through the bridge which can be done in network configuration on the host. In the second case you similarly have to configure the proxy or network server to allow you to access http files. There are dirty fixes for that but I will not recommend them. In the case of an institutional server you will have to get administrator approval, and in the case of your cell allow http access for your specific instance.
Also if you are in a VM and running a firewall on your host you can try update with it disabled.
TC
OK. First a few questions. Is your Linux Lite system running as a VM, and if so is the VM host Windows or Linux? If not, are you connecting through a proxy server, or through an institutional network like a library, business, or school, or through a cell hotspot, in order to connect to the internet? In the first case you have to allow secure connections and http connections through the bridge which can be done in network configuration on the host. In the second case you similarly have to configure the proxy or network server to allow you to access http files. There are dirty fixes for that but I will not recommend them. In the case of an institutional server you will have to get administrator approval, and in the case of your cell allow http access for your specific instance.
Also if you are in a VM and running a firewall on your host you can try update with it disabled.
TC
All opinions expressed and all advice given by Trinidad Cruz on this forum are his responsibility alone and do not necessarily reflect the views or methods of the developers of Linux Lite. He is a citizen of the United States where it is acceptable to occasionally be uninformed and inept as long as you pay your taxes.