Others have given you the solution, but as to why this is necessary: a property is just syntactic sugar for a method.
For example, when you declare a property called Name with a getter and setter, under the hood the compiler actually generates methods called get_Name() and set_Name(value). Then, when you read from and write to this property, the compiler translates these operations into calls to those generated methods.
When you consider this, it becomes obvious why you can’t pass a property as an output parameter – you would actually be passing a reference to a method, rather than a reference to an object a variable, which is what an output parameter expects.
A similar case exists for indexers.