I think what happens is that it exports as a WAV file first, then attempts to convert the WAV file to MP3. For this, you need the correct codec(s) installed. This is what happened when I tried the same thing:

Attached Files (Click to download or enlarge) (Only available when you are logged in)
2015-06-15_14-16-28.jpg (143.09 KB, 49 downloads)

BIAB & RB2025 Win.(Audiophile), Sonar Platinum, Cakewalk by Bandlab, Izotope Prod.Bundle, Roland RD-1000, Synthogy Ivory, Kontakt, Focusrite 18i20, KetronSD2, NS40M Monitors, Pioneer Active Monitors, AKG K271 Studio H'phones