So I have a engine light on my 2007 vw rabbit and cant get smogged.Smog shop failed it and said it was a spark plug cap but I had already replaced so what could it be.

Your check engine light is telling you there is a diagnostic trouble code (DTC) stored indicating something that is wrong with your vehicle. This code needs to have troubleshooting performed to get down to the root cause. I don't know what a "spark plug cap" is -- if there is a misfire, it could be spark plug, coil, fuel injector, or even base engine related. Testing is required.