Creating a trial of Dynamics 365 and Power Platform environments has always been complicated and subject to constant changes in the process and the Microsoft 365 user interface. I have documented the steps to create trials and you can download them from my OneDrive at https://tinyurl.com/trydyn.
There is now a new and easier way to create a trial environment in the Power Platform admin centre (https://aka.ms/ppac).
The new environment pane has a new option, Trial (Subscription-based).
There are other benefits as well – you can now create three trial environments not just one.
Note: You cannot create trials of Dynamics 365 Marketing in this way. If you want to start a trial for the Marketing app the follow the instructions at https://docs.microsoft.com/dynamics365/marketing/trial-signup
Read more on Microsoft Docs at https://docs.microsoft.com/power-platform/admin/trial-environments