i repaired avast and rebooted, but still, nothing will happen when i click send to virus lab.
OK, lets make sure that you are proceeding correctly first:
You open the chest, the file needs to be in the chest (you mentioned you couldn't delete it before) so did you actually send it to the chest instead of deleting it ?
If sent to the chest, you need to be right clicking on the file in the chest and select 'Submit to virus lab...' is that what you did ?