Sorry no idea what's wrong with that besides a network issue...you might need to post a log for that..all working fine here..
Cores can be downloaded as a full package from Retroarch addon settings or can be downloaded/updated individually from the Retroarch UI..
If you wish to try to download manually and try to run your roms before you solve the downloading issue, you can download from here the cores you need..
thanks, dont know how it can be a network issue - i can stream videos on Kodi and surf the net just fine.
Anyway where should i place the cores (running a RPI3B+) when i download them from your link provided?