Check your license terms:

Old trial remnants can cause conflicts. Use a clean uninstaller (like Revo Uninstaller) to scrub all entries, reinstall, then enter the key.

A serial number (also called a license key, product key, or activation code) is a unique alphanumeric string that verifies you have legally purchased the software. For Key Remapper, a typical serial number looks something like this:

KRM-3X9F-2A4B-7C8D-1E0F