Hi!
For sentinel demo environment you need a test tenant. You could use this - https://learn.microsoft.com/en-us/office/developer-program/microsoft-365-developer-program-get-started
To create sandbox tenant with E5 licenses. For MS Sentinel deployment test tenant is not enough.. as you need Azure Subscription. You coul either use your credit card and start trial Azure sub. for 30 days - https://azure.microsoft.com/en-us/pricing/purchase-options/azure-account
Or use Azure PAYG subscription with same credit card.. and test it for 30 days for 200$ as still sentinel is free for first 30 days.
Or you could also use Visual Studio (MSDN) Subscription if you have such licenses.
Here are good deployment what you could also use which will have data for investigation and training - https://github.com/SecureHats/Sentinel-playground