So Carla Rack has both Audio and MIDI I/O, but it’s not marked as instrument.
Hence by default (with “strict I/O”) the additional ports are ignored: A mono track remains mono, a MIDI track remains MIDI only etc…
In your case just disable “strict I/O” (context menu of the mixer-strip header), or create a track directly with “flexible I/O” or alternatively use pin-connections (plugin context-menu) to manually tweak things.