If you made a mistake when validating your Activity Report, don't panic!
It is up to the client to refuse your Activity Report so that it can be reopened. You just have to contact your client so that he does not validate the Activity Report.
Please note that if the last person on the Client validation workflow has already validated the Activity Report, you will not be able to modify it.
At this stage, for any modification, you will have to make a regularization on the next Activity Report.