Failed to schedule an activity whereas ressource fully available

Hello,
I have someone who work on a project "P1" at 100%.
This person have a training for 1.5 days during this project "P1".
Training activity have a higher priority than tasks on P1.
My problem is that I have a message when trying to calculate the planning for the "training" project, saying that activity can't be scheduled.
Indeed this person is planned to work on a tasks on P1 but what I don't understand is that this tasks can start before "training" activity and end after. It seems that planning calculation tool can not interrupt a task for some day to allow another one to be inserted ???
What have I done wrong ?








Thx

Hi Babynus,
unfortunately it does not seem to be this point, Michel is assigned to "formation" between 06 to 07, see below. What am I missing ?

I made tests on affectations, ie remove end date for "formation" project (no start or end date on other porject), and now the task is planned but it's not what I want to plan as "formation" is at fixed date (06/07 to 07/07), I want that other task on project "etude carte" start before this "formation" and remaining work end after "fromation".
See below "monthly planning for michel " before test: