Are you using a pi2 or pi3?? If pi3, there is a procedure in the documentation to disable Bluetooth and enable ttyS0.? The docs says that you do not need to run the procedure if you have a pi2, so I would guess that you do not need to disable Bluetooth with a pi2.? I recall that in a pi3, Bluetooth and ttyyS0 share the same resources which explains why you need to run the procedure.
I did the something and the serial UART would show communications with the TNC-Pi. ?Then when I ran YAAC and selected the serial port I would get garbage. After reading I discovered Bluetooth was using the serial UART. ?This explains the issue so did you turn of Bluetooth before installing the TNC-Pi?