You must delete the assignment to the activity.

But you should not have been able to delete the resource.
There is a control to block this deletion if assignment exists.
How did you do this ?

We installed Project'or recently to evaluate it.

Somebody used it before me but he left the company. I don't know if he deleted the resource and if so, how he did it. But the fact is, I have an activity with a resource assign to it but the ressource is unknown. If I clic on the resource name the resource page open with the message "Aucun élément ressource sélectionné". If I clic on the pencil, the assignation box open and I can change the fields "Taux", "Travail restant" et "Travail planifié".

Do you have some real work on the assignment ?
If not, you should be able to delete the assignment (or maybe an admin can do it)

If so, here is what you have to do, in the database (for instance with phpMyAdmin) :
=> on the application
- note the id of the activity (let's call is #idA)
=> using db tool
- delete all lines of table 'work' where refType = 'activity' and refId = #idA
- delete all lines of table 'assignment' where refType = 'activity' and refId = #idA
=> on the application
- back on the activity screen : se the assignment has disepeared, but work figured may be wrong
- create a new assignment (no matter the work or resource assigned) and delete it just after : the work figures should be correct now

Ok

merci !