Complex sentence: The female solo had not been written for Maarja-Liis Ilus in the first place but for another popular singer, Evelin Samuel who had to turn the offer down for an upcoming tour in Japan.
Split into simpler sentences: The female solo of the song had not been written for Maarja-Liis Ilus in the first place but for another popular singer, Evelin Samuel. However she had to turn the offer down for an upcoming tour in Japan.