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Quick summary:
Create a class by right-clicking in the Content browser - new C++ class
In blueprints create a source class (an empty class named source), generate visual studio project files and reopen the editor. Create a class you want and to remove the error after creating the class, remove Intermediate and Binaries folder and reopen the editor. If the solution loads incorrectly, open it manually.
UPROPERTY() is a way of declaring objects with a variety of options, which can change its accessibility, the way of editing, ...
Here's the link to all UPROPERTY() options of them: https://wiki.unrealengine.com/UPROPERTY