Sudo apt-get update not working

Ubuntu 19.04 is EOL or end-of-life (

Your issue is 'disco' (19.04) is EOL. After a release goes EOL, mirrors can drop it, and the main gets moved to old-releases (

A correctly upgraded system will have had those entries removed or commented out, but 19.04/disco upgraded to 19.10/eoan, as that was the next release so you possibly didn't use intended upgrade tools, or those repositories were re-enabled (and were on your system for a long time disabled

Personally I'd remove (or comment-out/disable) them, as they potentially open up security holes in your system, by allowing the installation of EOL or unpatched software thus potentially lowering your system security.