Why am I getting the error failed to expand /data/lists/lists/${lc for require_files: ${lc is not a known operator for this setting:

require_files = MAILMAN_HOME/lists/${lc:$local_part}/config.db

The value of require_files is a list in which each item is separately expanded. In other words, the splitting into items happens before the string expansion. You need either to double the colon, or switch to a different list separator.