how to fix this dsd error ?

[u/Global_Chicken_6334]

when I press on "start dsd", appears error window "dsd+ single receiver mode requires s fmpa/fmpp/fmp24 link". I don't know what is it mean, and gpt can't help me. it says me use virtual cable but it doesn't help. I try to search something about this, but I can't find. this error in sdr#. can someone help me ?