Garageband - MIDI Keyboard Creates No Sound

Hi,

I've been trying for several days to record some music with Garageband (v10.3.2) through my Tie Studio Midi 1i1o interface but I cannot get any sound.

I connected my Roland ep.75 digital piano MIDI output to the MIDI In end of my interface. The USB side of the interface is connected to a USB to USB-C adapter which in turn is connected to my MBP.

The MIDI connection definitely works since Garageband displays "1 MIDI Input detected" when I connect the interface. The old keyboard sends signals and Garageband detects them, because the blue MIDI In light appears when I strike a note. I created a green software instrument track but when playing the piano I am not able to create any sound.

There's also no problem with my output since I can play any other instruments, playing on the same track with the digital keyboard works, and recording using the same interface connected to my Yamaha DTX Explorer also works.

When I record the piano, little grey X symbols appear on the instrument track when I strike notes. They don't generate any sound. Also the volume bar of the instrument track never lights up.

What am I missing here?