Activation Error. Sim Card Not Supported for a iphone6s

I got cricket service . The iPhone 6s is under a contract but is no longer being used . I’m trying to insert Sim from my carrier . But it says not supported . I contacted the providers support group for this phone and it has a lock for activation with the provider and they need a request from the owner along with my email that I’m trying to activate with my service . And to remove device from contract . Does it need to be done like that or can y’all help me ?