What role do you need to have in order to be able to add a backend pool and a health probe to Azure Load Balancer?

To add the backend pool to the load balancer resource, the user needs to have permissions to be able to read the virtual network and virtual machine resources that need to be associated to the backend pool. Hence permissions need to be given at the resource group level.

To read the virtual network and virtual machine resources, you need to have at least Network Contributor role. You need the same role to add a health probe.