when I tried to load my hdl file to the program, the feedback at the bottom indicated out1[0..15]: sub bus of an internal node may not be used. Does this mean that I should do the 16 ways separately instead of combining them as a bus?
You don't need to say "a=a[0..15]". "a=a" means connecting all 16 bits. The error you are getting is because internal buses can only use  notation when they are connected to inputs. Just use "out=out1" to create an internal 16-bit bus named "out1".