It’s throwing an error before it gets to the real error message, so it looks a bit wonky. I’ll need to fix that up, but what it’s trying to say before it errors is that you’re missing a guess for a complete definition of the system. With adding the missing guess:
prob = ODEProblem(sys, [], (0,1), guesses = [sys.orifice1.port_b.P => 0.0])
sol = solve(prob)
It fails in the initialization with a CoolProp error:
julia> sol = solve(prob)
ERROR: CoolProp: unable to solve 1phase PY flash with Tmin=59.749, Tmax=3000 due to error: Inputs in Brent [32745.371466,47323.200000] do not bracket the root. Function values are [-1.000004,-0.999337] : PropsSI("ISENTROPIC_EXPANSION_COEFFICIENT","P",1.562500001e+12,"H",418309.2182,"AIR")