The uTrack24 have no gain level adjustement on the inputs, and the inputs can handle maximum +20dBu.
The Mackie Onyx 1640i outputs are +21dBu (post gain).
As I seen the mixer have +20dB headroom on the inputs. So that’s mean if you set 0dB as input level with the channel gain control, you’ll have around -20dB signal on the uTrack24.
Do you able to check the uTrack24 input level in channel focus mode eg. on the bass channel during recording?