When creating a new environment in Power Platform Admin center, you might see an error message coming up. This environment can’t be created either because the tenant lacks an active trial subscription or the limit of trial environments has been reached.
For creating new environment, you would need following plan.
Dynamics 365 Customer Engagement Plan
For learning purpose, you can take up trial plan. Register here using your Microsoft 365 account. You must be an Admin to create a free trial and assign it to your users in the organization.
Choose -> No, continue signing up.
Follow the sign up process. At the end, it will take you to the Microsoft 365 Admin Center. Click on Billing -> Your Products. You will see that a new product is added.
Now, go ahead and create a new environment in the Power Platform Admin Center.