You can see the entire error message by running the test in the cmd.exe/terminal window
In HDL file D:\TECS\projects\01\test\Mux4Way16.hdl, Line 8, a: the specified sub bus is not in the bus range: load Mux4Way16.hdl
The problem is that 16-bit buses have wire numbers 0 through 15. This is like an array that only allows indexes 0 through 15.
You need to write either a[0..15]=a[0..15] to connect all the wires, or you can simply write a=a which defaults to connecting all the wires in the buses.
Please edit your post to remove the otherwise working HDL.