The Arc is designed to be connected directly to the HDMI ARC port on a TV or projector, not a receiver.

And technically, for sound, the ARC on a receiver is probably an input, not an output - designed to receive audio from a TV, not output it to a soundbar.

It's actually a output on my Onkyo Tx-nr6100

I have a old plasma tv with no arc output. 

eARC stands for Enhnaced Audio Return Channel….meaning that the audio is actually transmitted in the opposite direction that normal audio and video over HDMI.  In other words, the eARC HDMI port on you receiver sends out normal audio and video to a TV or projector and can receive audio back from the TV or projector (such as streaming apps built into a TV)

So no, your receiver cannot tramsit eARC audio to  Sonos Arc.  There are devices that convert normal audio to an eARC signal though, many of them with HDMI switching capabilities.  HD Fury, Feintech, etc.  @Ken_Griffiths post a good list earlier today.  I’ll see if I can find it.