If you are working in a Sandbox environment and you are trying to push a Change Set to your Production environment, you will first need to Allow Inbound Changes to your organization's environment.
Log in to your Production environment
Navigate to Setup > Deploy > Deployment Settings
You’ll see a list of your current sandboxes, and if they are authorized to push change sets or not.
Click Edit next to the one you want to allow and check next to ‘Allow Inbound Changes’, then Save.