So I'm trying to follow the instructions from the old reddit community to build the miryoku firmware for [seeeduino_xiao_ble, chiper36, Colemak, default, Mac, flip, default, default, default, default].
The branch selector in the workflow is for choosing which Miryoku ZMK branch to build from. Unless you've made your own branch in your fork or you've manually sync'd an experimental branch from upstream, you'll only have the main branch in your fork.
The branches field in the workflow is used for selecting and merging ZMK forks and branches. That's what you need to do here, so put zmkfirmware/zmk/main in the workflow branches field.