Ah, okay. For Dynatrace SaaS, the option to split DDUs across environments is not currently available - but is planned to be implemented.
However, you are still able to view DDU Overviews within each environment under the 'Accounting' drop-down in Settings, as well as configure DDU pools to limit the consumption within the environment. You need the Change monitoring settings permission or use an admin account to access the DDU consumption overview page.
More information on this can be found here: https://www.dynatrace.com/support/help/shortlink/davis-data-units#ddu-pools