gkaiseril is suggesting that you may have a file explorer open. The Preview feature of the file explorer will cause the issue you are having.
Another cause is trying to save the stamp file to the Plug-ins stamp folder, which is the case here. The plug-ins folder is a protected area. And to make matters worse, Acrobat opens these stamps files and keeps them open. So you cannot write back to a stamp file in this location until Acrobat is closed. Just save your stamp to a temporary location, close Acrobat, then move the stamp file.
I would strongly suggest that you save your stamp in the User Stamps folder to avoid this issue. If there is a copy of your stamp in the Plug-ins stamp folder, you will need to remove it.
Learn more about PDF stamps here:
PDF Stamp Annotations