Can't get digital out to work through receiver

Saw someone on YouTube using both RCA and coax digital out at the same time, but I can’t get it to work.

I have RCA connected to CD input on a receiver, and it works fine, but when plugging the coax out to one of the digital inputs I get nothing. Using a coax ( orange ended) cable.

DAC is connected to a Mac, if that matters.

Any ideas what I am doing wrong? Thanks

I posted this under “general”, but was suggested I post here for help.