Unable to move files to archive storage in Azure Storage Account

I'm struggling to move some files/folders from the cool tier in a container into archive storage. I've a load of machine backups that I want to get migrated over.

I've got the files in a container that's set to private and I know I need to select each of the files individually and change the properties to archive.

The problem is that when i go into the properties of the file, hot an cool (inferred) are available but archive is greyed out.

Anyone any ideas? Followed a couple of articles below but no matter what I'm unable to get them over to archive storage.

https://www.techtalkcorner.com/move-files-archive-tier-azure-storage/ https://docs.microsoft.com/en-us/azure/storage/blobs/storage-blob-storage-tiers?tabs=azure-portal