Thank you for reaching out and I am happy to assist you with this.
In order to change the settings on subsites, the network admin must uncheck the “Use single network configuration file for all sites.” setting in Performance>General settings>Miscellaneous and save the settings.
After that, you need to go to the subsite dashboard, and in the Performance>General settings click on the button that says “Use specific settings”. Once that is completed navigate to the Performance>Page Cache.: