Purchase Order blocked by the system


I am a Dynamics AX Administrator. One of my user as a problem, he is in the Purchase department, he is one of the people who validates the Purchase Request in his department. Here is the problem, when someone makes a Purchase Request and he is the one who validates it, the request followes the workflow up to the end, but when it is supposed to become a Purchase Order, the system blocks it and there is an error message saying that the PO has been cancelled. The other person that validates like him doesn’t have that problem. When she validates a Purchase Request, this PR goes all the way to the end and becomes a PO. I checked their rights, and everything seems normal. I don’t know where the problem is coming from because even when that user takes another computer and use his session on it, every PR he validates ends up by being blocked when it is supposed to become a PO.

Thank you in advance for you answers.