Do you know what version of the beaglebone black you have ? If it's 4 or 5 years old, the eMMC may be too small to fit the Bela image. There are workarounds there if needed.
However, the most likely cause of failure is just bad luck, so can you try the process again?
Not sure what you mean by "bela 2ch kit", but if it's the Bela Starter Kit, which is a Bela Cape and a Beaglebone Black, then there is no need for an SD card, because the eMMC is already flashed. Have you tried booting it without SD card?
What do you need the source of supercollider for? Building supercollider for bela is done from a different repo (github.com/BelaPlatform/supercollider) but the version that ships with the board is already the most up to date one. I need to update the repo when I have some time, but that's also not straightforward.