Hi @EllaHuang,
Do you want to create a DLP policy in your default environment of Microsoft Flow via Flow Admin Center?
I have made a test on my side and I could set a DLP policy in my default environment of Microsoft Flow.
If you want to set a DLP policy in your default environment of Microsoft Flow, please make sure that your login in account of Microsoft Flow is a Environment Admin role. In other words, your login in account of Microsoft Flow is an Environment Admin user of your default environment.
More details about setting an Environment Admin role for environments, please check the following article:
Manage your existing environments
Best regards,
Kris