This property can be changed only by using the VMware VirtualCenter Server to change the host's maintenance mode setting. ...

This property can be changed only by using the VMware VirtualCenter Server to change the host's maintenance mode setting. Putting an ESX Server host into maintenance mode sets the AvailableForPlacement property in VMM to false, and taking an ESX Server host out of maintenance mode sets the AvailableForPlacement property in VMM to true.