haK Is it possible that the Bela boot system is not flashed onto the internal memory?
yes that sound right. The kit should have come pre flashed with the Bela image but maybe something went wrong when we put it together. If you have a spare SD card around that you could temporarily put to this task, you should be able to follow the instructions at this page to flash the SD card, then plug it into the BeagleBone and reboot the board. As mentioned by @Remork, you should hold press the USR button while powering it up, however that tends to be complicated and hit-and-miss with the Bela cape on top, so I normally recommend following Option 1 from this other page. Once the board has booted successfully from the SD card and you can access the Bela IDE at 192.168.7.2 (7, not 6), or bela.local, in the console at the bottom of the IDE enter this line and hit enter:
/opt/Bela/bela_flash_emmc.sh
The LEDs on the board will turn solid blue and the process of flashing the embedded memory (emmc) will start. Once the LEDs go off and the console shows a success message, you can gracefully shutdown the board, remove the SD card and reboot it from the eMMC. If everything goes fine, you no longer need the SD card for Bela and you can reuse it for whatever you need.
If you don't have a spare micro SD card to use for this purpose, please contact us at orders@bela.io. Apologies for the inconvenience.