Is your license for an older version of FlashBack?
FlashBack licenses are associated with a particular version of FlashBack so will not be accepted by another version. If you have license for FlashBack 1, 2, 3 or 4, it will not be accepted by FlashBack 5.
You can download the correct version of FlashBack by logging into your account and going to the Download Products page - the download links provided will be the appropriate ones for your license.