Can someone please tell me what this means and how to fix it as it is not telling me what is wrong in the script?

An assertion is a test to check for something that should never happen, and in the event it does happen it will exit the program to prevent further loss. For example, a key component of the engine being null would mean nothing can continue, so it fails. But I'm not sure why it would happen on a script problem, as usually it will give you some helpful error message. More likely it is an issue with the assets or some sort of corrupted resource that is attempting to load. Maybe if you can provide the exact text of the error, or some screenshot, or further information about how the problem happened (what was the last thing you did) maybe that would help.

a year later