Hi @Zhukov23,
Do you want to list user's Planner tasks within an app as a admin?
Further, do you want to list a specific user's Planner tasks within a Planner plan?
If you want to list a specific user's Planner tasks within a Planner plan, I have made a test on my side, please take a try with the following workaround:

- Add Planner connector and Office365 Users connector as data sources within my app.
- Add a Dropdown control (Dropdown1) within the screen of my app, set the Items property of the Dropdown control to following formula:
Planner.ListMyPlans().value
- Add a Gallery control within the screen, set the Items property of the Gallery control to following formula:
Filter(
Planner.ListTasks(Dropdown1.Selected.id).value,
createdBy.user.id=Office365Users.UserProfile("Test3@xxxxxx.xxmicrosoft.com").Id
)
Note: The 'Test3@xxxxxx.xxmicrosoft.com' represents the email address of a specific user within my Org.
On your side, you should type the following formula:
Filter(
Planner.ListTasks(Dropdown1.Selected.id).value,
createdBy.user.id=Office365Users.UserProfile("The email address of a specific user").Id
)
Best regards,
Kris