Can you please let us know what error message you get when you convert your Flac file to MP3?
Some of the reasons could be if the Flac file has codecs that is not compatible with RealPlayer.
As a workaround, convert the Flac file to WAV and then convert the WAV file to MP3. This will ensure that the file is converted to MP3 if Flac to WAV is successful.
Anyway, please let us know what error message you get.