Each major version of Duplicate Cleaner will have a different license key.
Duplicate Cleaner versions 3 and 4 have the following format:
XXXXX-XXXXX-XXXXX-XXXXX-XXXXXX
Duplicate Cleaner 5 has a longer format which isn't split into sections. It is recommended to copy and paste the key into the program:
UHEGfu==-GafgafgirRYRYrag4Hj5757557oijdhD5hdth+5SRYRdgyjDfbXBVBVXVBrghdthdtjdtjtu53hSfhSFHJTtT564hHH
If you are having trouble, check you have the license key for the correct version. If in doubt, copy and paste the key. You can copy the key by selecting it with the mouse and pressing CTRL-C. To paste it into the program use CTRL-V in the license key box or use the Paste button if present.
If you are still having trouble, please send a screenshot of the program with any error messages to support.