It seems ModelingToolkit doesn’t accept a scalar for pmap, so you need to define pmap with a comma at the end to make it a tuple containing one Pair and not a scalar Pair:
pmap = (T => 300.0,)
Note also that sometimes you may need to register your custom function with Symbolics.jl (though not in this case). You can read about that in the link given in the Catalyst FAQ.