It’s designed for an NC2/3. Their bumpers are slightly different.
I had mine fitted for me by a specialist. Basically that black valance (the plastic looking strip) you already have is part of the bumper. In order to get the new diffuser on, they had to cut the bumper roughly up from there anyway. TBH I’m happy I got a pro to do it!
@oldmanbigkid has done a great job on his. See: BOFI rear defuser fitting to NC1