As I said it was a silly mistake, The API sent me the resopnse in the http format instead of json.
import groovy.json.JsonSlurper
import com.eviware.soapui.support.types.StringToStringMap
import groovy.json.JsonOutput
responseContent = testRunner.testCase.getTestStepByName("CreateGroup").getPropertyValue("response")
def confId = responseContent
log.info confId
testRunner.testCase.getTestStepByName("DeleteGroup").testRequest.setPropertyValue("groupId", confId)
This code works for me now, I'm sorry if i've wasted your time, but I'm new with this environment and the message wasn't super helpfull, but I've learned my lesson.