wowpedia shows that that function has been moved to its own data table in 9.0
C_CurrencyInfo
Now whether just adding C_CurrencyInfo. in front of the function is enough I don't know. But worth a try.
https://wow.gamepedia.com/API_C_Curr...rrencyListSize
Seeing as the addon creator doesn't seem to have logged into Curse in two years you may have to adjust the addon yourself if you still need to use it.