When you install Device Manager, the Avast Business Antivirus client is included as part of the installation package. However, Avast Business Antivirus is not activated until you apply a configuration policy to the Device Manager that determines which components to install, which scan settings to apply, etc.
You apply configuration policies to a Device Manager the same way you would to other devices, either by manually selecting the device, or creating automatic application rules that include the device. See Manually Applying an Antivirus Policy to Devices and Groups and Creating Rules to Automatically Apply an Antivirus Policy.