“Protocol Unknown” error in 1996 Impala SS

Hi there,
“Protocol Unknown” is the error message I am getting. What meaning does this have? This happens when trying to patch into the PCM. A 1996 Impala SS It 1 w/ 4160e is what I am trying to patch in. A Serial and UBS scan tools have been tried but with the same results. Any solutions can I troubleshoot and test out?
