"Collectivism" is a simple concept, and concepts are what we use within our minds to reference things in reality. Collectivism describes a process by which we take an attribute (or several attributes) of an object that is included in a collective, and pretend that the attribute(s) apply to the collective itself.
The error:
of an object → to the collective
or more generally, collectivism is an example of the error:
of the instance → to the non-instance
It's just that simple and the consequences of the error are huge. It's not merely the misidentification of something, as we do that regularly. It's the misplacement of an attribute or attributes in such a way that it will lead us to completely mis-identify the nature of so many things around us, our own nature chief among them. As should be obvious, Collectivism is the source of "identity politics" and the belief that we are not Individuals but rather mere units of some larger collective.
The error was never that the collective isn't there. We are a society, trees do comprise a forest, and so on. Averages, statistics and generalizations are all legitimate (true) references. The error is evading that they are collective concepts and applying the attributes of those generalizations to the instances subsumed within them. With the use of language, such errors can run deep within our minds. It's an old saying, "He didn't see the forest for the trees," but that was never the problem. The gross error was that because we have the concept for a forest, we fail to see the trees for the forest. It can be a deadly error. No person was ever crushed and killed by a forest, they were crushed and killed by a tree. The attributes of a collective concept are not the attributes of the objects subsumed within them.