This happens if the c++ class is renamed and you forget to add a Core Redirect, which is why the Blueprint system will not be able to find its parent.
This situation is pretty much always explainable and simple to solve.
If it’s a Blueprint class complaining about its parent class which is also a Blueprint class, you might be dealing with some form of corruption. In that case, if the editor does not allow you to set a new class you could attempt to by using this tool: