(sorry, posted this some time ago but email didn’t get through)
hm, instructions work fine here with 2021-05-07-raspios-buster-armhf-lite.zip on a RPi3. i could compile SC and didn’t get your missing X11 error.
perhaps try again and take extra care only running the “For a GUI-less build” sections. (i know the gui vs non-gui parts are a bit mixed up - could be improved).
also, sidenote, if you’re planning to use the built-in or HDMI audio outputs, i’d recommend to for now use an older disk image (before dec2020) because something got messed up when Raspberry Pi OS switched to PulseAudio. see https://github.com/supercollider/supercollider/issues/5480
so for example a 2020-08-20-raspios-buster-armhf-lite image will work well. with newer images you’ll run into issues with jack vs pulse.
though for usb soundcards this isn’t an issue. then the latest 2021-05-07 image is ok.