Why does the Acoustica ARA plug-in show "The Acoustica ARA client requires an ARA2 compatible host application"?

You will need an ARA2 compatible host application (see https://en.wikipedia.org/wiki/Audio_Random_Access for a list of compatible hosts). In some hosts, like Logic Pro X you can choose both the normal and ARA2 version of the plug-in. Please make sure to choose the ARA version.