A bit late, but for the benefit of anyone else finding this:
Note that you also need to request read/write access to the device in your .adef file - or run unsandboxed.
See: How to send and receive data using UART2 (RS232) interface - #3 by raf
it would be nice to have this piece of information (i.e. that le_tty is only available on 16.10 and onward) on legato.io, directly where le_tty is documented.
Indeed!