Complex sentence: The band was the brainchild of the members when they wanted to write pop, almost bubblegum pop, music, but none of them could previously play the instrument they played in the band, which added to their amusement.
Split into simpler sentences: The band was the brainchild of the members when they wanted to write pop, almost bubblegum pop, music. None could previously play the instrument they played in the band, which added to their amusement.