A Public IPv6 Address can be attached to Load Balancers to the Primary Network Interface, or a Secondary IPConfig of the Network Interface of a Virtual Machine. You'll need to have an existing IPv4 Config, the IPv6 cannot be a Primary at the moment. You'll also need to make sure the Region of the IPv6 matches the Virtual Machine/NIC.
https://learn.microsoft.com/en-us/azure/virtual-network/ipv6-overview#limitations