display3958023

@display3958023 The reason is simple. All in one installer is available only for Windows.  Information is available in the help of ESET Remote Administrator: https://help.eset.com/era_admin/65/en-US/deployment_scenarios.html?fs_local_deployment_aio_create.html
In case of a mac product, you can either generate an agent live installer script, or deploy the agent installer manually. Installation of the security software product can be then performed using a software install task. 
PS: I would strongly recommend to upgrade your server to ESET Security Management Center V7, which was released more than a year ago. 

You can download the standalone mac endpoint installer at eset.com (download section). Specifically here: https://www.eset.com/int/business/endpoint-antivirus-mac/download/ 
Installation works in the way, that when you choose a product you want to install, agent will connect to ESET Repository (cloud download server), and will download and installed the respective product. You can cache installers by a proxy server placed in between, to optimize a network traffic. 
With regards to the appliance upgrade, instructions are available in the documentation: https://help.eset.com/esmc_deploy_va/70/en-US/va_upgrade_migrate.html