I cannot open the character model?

it also won’t let me create any new models

Sorry about not explaining well, I was just frustrated. Any time that I open Unity and click on an icon, a error appears at the bottom saying “Unable to open Assets/Standard Assets/Character Controllers/Sources/PrototypeCharacter/Constructor.FBX: Check external application preferences”

It wont open unless you have an application that can do that.

If you have used Blender to export that .FBX , then you cant just open it back with Blender, instead you would have to import the .FBX into blender.