if I get you correct, you're unable to add hosts to the session pool of AVD?
If this is the case, this error might be showing up because the provisioned VM isn't capable of accessing the storage blob and downloading it. Typical reasons for this are:
- Network Route Table is not correct
- Firewall in front of VM and no allow rule
- Network Security Group of Subnet prevents VM from accessing Internet outbound
Please make shure that your network is setup correctly and the VM is capable of communicating to Internet outbound. To check this, you could use the Network Watcher https://learn.microsoft.com/en-us/azure/network-watcher/network-watcher-connectivity-portal to check connectivity.