This error occurs when you have already used your license key in the past.
All Premium Version licenses to use the DLL-files Fixer are to be considered personal, for one person and computer. However, since you might need to upgrade hardware or replace your computer before the license expires, we allow for additional installations, as follows:
1 Year license: 3 unique installations.
2 Year & 3 Year licenses: 5 unique installations.
Every time you use the license to the DLL-files Fixer on a new computer it will count as a single usage.