Fix mixed model failed to authorize - Missing or invalid customer key - Simulation in Proteus Demo

If you are new to Proteus and you are facing a problem. You Just received an error message for a simple LED blinking program. The error message is as follows: mixed model failed to authorize - Missing or invalid customer key. That is because you are using a Demo Version of PROTEUS, nothing is wrong with your model or code that you designed in ISIS? Here is how to fix this Educational Engineering Channel