Hi @TonyG22
Make sure you have installed the "Dynamics 365 Customer Service Community" App in the proper environment. You can see the confirmation in the Power Platform Admin center like below

Solution:
Once it is installed, then follow the official docs to get your "Modern Community" portal creation.
1. Go to https://make.powerapps.com/
2. Choose the correct environment where you have installed the "Community service App"
3. Click Create > under the start from template > You can see modern community template
4. Then provide the necessary details.

Note: While choosing a naming convention: Keep it as simple as possible. EX: SandboxRR. Later you can change the Title of the portal in the web page design.
Hope it helps.
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