Wrong operating system
Unless you purchase a universal license key, Reflector keys are operating-system-specific, meaning they only register the software on the operating system the key was purchased for. If your key is not registering the software, make sure you've purchased a key for the correct operating system.
Wrong version of Reflector
The original Reflector, Reflector 2, and Reflector 3 all require separate license keys.
Reflector 3 uses a new style of the license key. This key is much shorter than the original key for Reflector. These are separate license keys that are purchased independently.
If your license key doesn't fit in the registration box, it's likely you've got a Reflector key, not a Reflector 3 key.
New style keys follow this format:
Old style keys are a very long strings of random characters. These keys often exceed 100 characters.
If your registration window looks like this, you're using Reflector 1. Either a Reflector 1 license will be required, or you can download Reflector 3 from here.
Please note Reflector 1 will not work with iOS9 and above.
Reflector 3 registration window:
Reflector 1 registration window: