Hi partner,
Strictly speaking, there are four types of environmental types.
- Production
- Sandbox
- Trial
- Default

The types of environments that can be deleted are sandbox and trial.
Production and default environments cannot be deleted.
In order to delete a Production environment, you must first change it to a Sandbox environment and then delete the Sandbox environment.
Please refer to this document for Environment overview:
[View:https://docs.microsoft.com/en-us/power-platform/admin/environments-overview:320:50]
Please refer to this document for Delete an environment:
[View:https://docs.microsoft.com/en-us/power-platform/admin/delete-environment:320:50]