PCM mode is a digital audio file that should be sent to the soundbar via an optical or coaxial digital audio cable. The coaxial digital audio cable uses an RCA plug but not the L/R stereo channels of analog audio. The aux input is usually the 3.5 MM cable from a computer, CD or MP3 player or a cell phone or tablet.
First, which type of RCA cable is connected between the sound bar and the TV? Second, are you sure that the TV connection is an audio out? (A number of recent TVs have only one audio output. This means there is only the optical audio out and not even a headphone jack.)
Once you double-check the input at the sound bar, choose the Line In input if you are using the stereo RCA cable. If you are using the Optical input, select that input. Use either the Mode button or the specific input button on the remote. Then double-check that the TV is set for the appropriate audio out.
I hope this helps. Please add a comment with the model of the TV and the sound bar for further information. I looked at the ONB18SB001 documentation for some of this. The manual for this Onn soundbar is available at Manuals Lib. See https://www.manualslib.com/manual/1498038/Onn-Onb18sb001.html?page=12#manual
for the list of input modes and the input to which they correspond. Here is the image of the inputs on the back of the ONB18SB001: https://www.manualslib.com/manual/1498038/Onn-Onb18sb001.html?page=7#manual