Hi, I've tried to convert currencies according to this thread: https://www.reddit.com/r/kde/comments/6sk3ku/currency_conversion_in_krunner/ and it didn't work. For example '1000 USD in EUR' showed me nothing.
Weird, works for me in 5.11.5.
The converter extensions is on in the settings(for ex. it works with mass) - if that's what supposed to do conversion.
Can you check whether there's a valid and up to date currency.xml in ~/.local/share/libkunitconversion
If you can provide the information requested in comment #3, please add it.