I see anarchism as the set of all social organisations that are against the initiation of force as a means of social control.

I see capitalism as the set of all social organization that includes property rights (individuals or groups are assigned ownership of objects through some form of social norm or legal framework that extends beyond their individual or groups ability to control objects into the actions of other individuals or groups)

Neither anarchism or capitalism by these definitions seem to me to have opposites in my opinion since you have both the absence of them and the presence of alternate social organization: for anarchism you could have no agreement about force or you could have an agreement that force ought to be used to control people. For capitalism you could have no agreement or legal framework for assigning ownership of objects or you could have alternate forms of property for example collective ownership (although some here might argue that if the collective is finite then it is sill private property and so capitalism ie your collective is just a corporation but if ownership is universal then that seems the same as having no agreement) as well as the alternate arrangement assigning control of property over to something non human like an algorithm.

To be honest this sub is already a pretty good source for pro-government arguments and against capitalism as we tend to debate each other endlessly on everything from the philosophical foundations of our beliefs to how it is to be presented or whether we should even do that rather than focusing on something like agorism.