A couple of things to note during the channel upload:
1) SL commands needs to be enabled.
2) The radio modem must be in data transfer mode.
3) The serial interface baud rate, parity and number of data and stop bits must correspond to the settings of the radio modem. (When you access to the programming mode with SaTerm, these are always 9600-8-N-1, but in channel upload case, it can be e.g. 19200-8-N-1.) Please, double-check and confirm.