What you have is a free license. Hot add requires a minimum of Enterprise.
But I am only changing the number of cores.
What do I need to purchase in order to add more cores? The server has 1 CPU and 6 cores?
You can change the settings while the VM is powered off. Please remember that the free Hypervisor edition is limited to 8vCPUs per VM, no matter how these 8 vCPUs are presented to the VM (as sockets, or cores). Any paid edition will remove the 8 vCPU limit.