Selecting Inputs/Outputs In Garageband

To change the input and output source within Garageband, go to Garageband>Preferences> Then ensure that the relevant devices are in use. In this case, the Scarlett 2i2 has been chosen as the input/output device.
Garageband Select Interface 
To select or change which channel records into Garageband, create a new track and a dialogue box will appear. Select either “Real Instrument” or “Electric Guitar” based on what you are recording. You can choose input/output, which at this stage should be your interface. 
 I/O Select
Then by clicking the dropdown box “mono 1” (highlighted) you will be able to select the relevant input.
Specific Input select
Alternatively, if you are using a Real Instrument track you can change I/O settings by clicking the Master Track (top right corner) and then changing the options in the bottom right corner (highlighted) 
Alternate I/O Select