Message "This Serial Number in not valid for Adobe Photoshop Elements 15"

I purchased an upgrade version of Elements 15, which I intended use to upgrade my existing Elements 12. I ticked the box to signify that I was upgrading an earlier version. unfortunately as it commenced the download there occurred an interruption which stopped all communication. When I managed to get back on-line I had to download a trial version, but when I tried to use the new serial no I received the message "This serial Number is not valid for Adobe Photoshop Elements 15". I then tried entering the previous serial number but received the same message. Where did I go wrong?