Fetching non-existent fabric loader?

0.19.4... Prism and Fabric only recognize up to 0.19.3

Fyi this was on the web version of the installer if that makes any difference

I encountered this issue too; fabric 0.19.4 does exist, but the Prism Launcher metadata updating script seems to be broken right now. I have already reported it to the Prism Launcher maintainers, but we can't really do much else but wait for them to update it.