nuno-miguel-lourenco

You have the answer on the exception trace:
A message body reader for Java class com.jaspersoft.jasperserver.dto.authority.ClientUser, and Java type class com.jaspersoft.jasperserver.dto.authority.ClientUser, and MIME media type text/plain; charset=UTF-8 was not found.
The registered message body readers compatible with the MIME media type are:
text/plain; charset=UTF-8 ->
You must make a request with an appropriate HTTP Header - Content-type despite of the REST service you're invokingMIME type text/plain is not a viable one. It should be:application/xmlapplication/json

As you are referencing a Variable, I would say that $V{UnitPrice} is NULL, and therefore your final result is also NULL