make something up

"Make something up" is a command given to someone when you want them to make an excuse so others won't be asking any more questions. "He can't make something like that up!" is said when you know someone is speaking the truth because the information or reasons given given by this person are too complex or indepth to be dismissed as a lie; therefore, it is assumed to be true. "You can't just make that up!" would mean that the person you are speaking to should not be foolish enough to think he or she can lie about (a specific) something and then expect to get away with it; it won't be credible. Or perhaps it would be credible, but it would be considered unfair. Whether or not it is credible would depend on the situation.