There was some task with a validated duration of 0. Wich is normal if the task is "planification" for example. So I solved my problem but there is still an issue. At least the error message should say where is the problem (which task).

This isn't an error.

You use the planning mode "Fixed duration" who forcing you to set the duration date.

If you want don't see the message anymore, change to a floating planning mode.

May be the "project template" feature can be improved for fixed planning modes.

thanks for your answer.

I can set the planning mode on a task but I can see where you can set it on the project?
Because there was no error on tasks. Only on the project when I tried to copy it.

I can set the planning mode on a task but I can see where you can set it on the project?

There is no planning mode on project, only on activity.

Because there was no error on tasks. Only on the project when I tried to copy it.

The error is on the task. What is weird is that you should have the error on the template...
Can you try and update every activity of the template to identify source ?

I did another try and I have the same message. I refreshed everytask, replanified, and still no task in error. This time again I have a validated duration at zero, to planify as soon as possible, with a planified duration of 1 wheras the begin and end date are the same.

Finaly it was another task with no duration validated (no mistake again, and it's normal). When I set one, I was able to copy.