Activity priority

Please be more precise.

I can see
ATB 1
- ATB 1-1
-- ATB 1-1-1
ATB 2
- ATB 2-1
-- ATB 2-1-1

Gestionnaire is allocated as Project Leader to ATB 1 and ATB 1-1 (no use as prfile is inherited from parent) and ATB 1-1-1.
Gestionnaire is allocated as Project Leader to ATB 2 and ATB 2-1 (no use as prfile is inherited from parent) and ATB 2-1-1
Dev ATB is allocated as Projct Member to ATB 1-1-1 and to ATB 2-1-1

ATB 1 and ATB 2 are NOT sub projects of JES, The are at root os structure (wbs 84 and 85).

i want to create activity to Dev ATB on ATB 1-1-1

The is no activity on ATB-1-1-1...
Connected as admin, I just created Activity "New activity" on project ATB 1-1-1, then assigned Dev ATB : all works fine.
Connected as Gestionnaire, I just created Activity "New activity as gestionnaire" on project ATB 1-1-1, then assigned Dev ATB : all works fine.
As supervisor, you cannot create activity nor assign resource : supervisseur only has "read-only' rights.

Please explain what you try to do, expected result and what you get.
Please post screenshot to explain.