FLAC has nothing to do with FL Studio. Logic Pro X does not export to FLAC, either, for example. Generally an in-between program (possibly a mastering program) is used for such conversion.
Any sequencer, including FL Studio can export to an uncompressed format such as WAV or AIFF, which are native, uncompressed formats.
This is the highest possible quality (depending upon the settings of the export–resolution, sample rate, etc).
From there, that "master" file (which may in turn actually be "mastered"–another topic) is rendered into formats suitable for sale and download, such as, and including, FLAC, MP3, ALAC, etc, which are much smaller in file size, and typically sold/sent to the listener (and the online store).
This may help