You can only attach virtual machines that are in the same location and on the same virtual network as the LB. Virtual machines must have a standard SKU public IP or no public IP.
The LB needs to be a standard SKU to accept individual VMs outside an availability set or vmss. VMs do not need to have public IPs but if they do have them they have to be standard SKU. Vms can only be from a single network. When they don't have a public IP they are assigned an ephemeral IP.
Also, when adding them to a backend pool, it doesn't matter in which status are the VMs.
Note: Load balancer and the public IP address SKU must match when you use them with public IP addresses.