The financial market prices that can be pulled into Numbers using functions come from Yahoo Finance. You can only bring in market prices to Numbers using a financial function only if they exist in Yahoo Finance.
To check if historic prices exist in Yahoo Finance, go to the Yahoo Finance web site, punch in the symbol for the currency pair you are looking for in Yahoo Finance's search bar (CAD/RUB for example) and see if it exists. If it does, then check if there is data displayed in the Historical Data section for the symbol.
In case historical prices do not exist in Yahoo Finance for a currency pair you are interested in such as CAD/RUB, a workaround is to derive them from the respective USD quotes for each currency if they exist in Yahoo Finance. For example, pull in USD/CAD and USD/RUB using CURRENCYH, and then divide the latter by the former to get CAD/RUB.
In addition, the CURRENCYH function pulls in the historical market closings and only starting after midnight US time for the latest closing. You can pull in the current day's quotes with a bit of delay using the CURRENCY function.