The casting expression cannot be completed, because you cannot convert to or from the specified type.
For example:
class-id myClass1.
method-id myFirstMethod static.
01 myItem1 string.
01 myItem2 type myClass2.
01 myItem3 object.
set myItem2 to myItem1 as string *> this is incorrect unless type MyClass2 has an implicit\explicit conversion operator
*> from type myClass2 to string
set myItem1 to myItem3 as string *> this is correct - note: this could produce an exception at runtime
end method.
end class.