You should be able to edit DDU quotas for Dynatrace tenants, assuming the proper permissions are in place. I don't see any image attached, are you able to attach a screenshot to what you are referring to?
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
This functionality is eventually planned for release but unfortunately, as of right now, there isn't a confirmed timeline as to when it will be released