Hi dqninh,
this will never work because:
The SX900 has no built-in USB Audio interface. The USB-to-Host connection to a computer can only used for MIDI-over-USB. ("Audacity" cannot record MIDI but only Audio.)
The easiest way to make an Audio recording is to use the SX900's internal Audio recorder. You can then connect the USB stick on which the recording was saved to the computer to further edit the Audio recording.
If you want to make the Audio recording on the computer (e.g. in a program like "Audacity" or in a DAW), you have to connect the Audio outputs of the SX900 (Main Out L and R) to the computer (preferably via an external USB Audio interface). Alternatively, you could connect the audio outputs of the SX900 directly to the line input of the computer (with an appropriate cable "2 * 6.3 mm mono (TS) plugs to 3.5 mm stereo (mini TRS) plug". The latter can, however, lead to sound problems (e.g. hum / noise / distortion / poor frequency response). It is generally recommended to use an external Audio interface.
P.S.
It is recommended to first record your playing on the SX900 as a MIDI file. This can then be edited if necessary. Then you let play the MIDI file while recording it as an Audio. (If you want, you can also sing along with it at this time.)
Best regards,
Chris