To add a database to an existing PowerApps environment, do the following.
Log into https://admin.powerapps.com/ and go to Environments, then select the environment. You will see a Create my Database button. Click it:
You will see below:
Select a Currency and Language, then click Create my Database:
You will see “provisioning database”:
Once complete, it will look like below:
Go to https://make.powerapps.com/. Select the environment, and you will see the entities installed: