I built mine a couple of years ago. I don’t remember any errors with “make install”. I used the 64GB image (with the sound library) SD card:
64GB (with sounds) https://storage.googleapis.com/open-nsynth-super/images/onss_1.2.0_full.img.bz2
16GB (without sounds) https://storage.googleapis.com/open-nsynth-super/images/onss_1.2.0_lite.img.bz2

As I remember, you just have to record the image in the SD (via Etcher), put it in the raspberry and enjoy!