Penny Fowkes

I have a new 3rd gen Scarlette 18i20 that I am trying to use with Cakewalk. When assigning instruments to tracks, only the odd numbered inputs are available as "input1 left, input 1 right, input 1 stereo etc. and the same with 3,5,7,9 No even numbered inputs are listed in the drop down. What am I doing wrong?