The booking probably has your BA number in it, not your RJ number. Only the latter has status at oneworld Ruby level, the former is presumably BAEC Blue. It used to be possible to switch the FFN out on the Finnair website, but I think that no longer works. rj.com might be another route - I'm not sure what functionality that allows. Alternatively you might find it easier to just call BA and ask them to switch the FFN to your RJ membership number.