Inserting external Comp

[ShebaStudios]

Hi,

I'm trying to route a recorded track through an external comp.

I have set the output for the existing track to 3-4, taken output 3 of my interface(presonus firebox) inputed it into my comp, output of comp into interface input 3(line in), created a new track, assigned it to inout 3 and put it into record mode. Signal passes fine and comp is working great. However when I go try to record I get "error 12345", any thoughts?

Secondly, is there a better way to insert the comp onto the existing track through a bus/aux? If so please advise.

Thanks so much.