Can you share the URL of the API endpoint you are using? My guess is that $plan.id$ is returning null, hence the error message. You can verify this by finding the request details in the ccg.log file on your VA. It should tell you what is in the request body.
Hello @apino ,
You should use $plan.nativeIdentity$ instead of $plan.id$.
Also, as you have put double-quotes around $plan.id$, you should ensure that the Web service is expecting a string and not an integer.
It really depends on which attribute you have chosen as the ID, the Web service, etc.