What could be the reason for a VM to be in tab Unsupported on the Just-in-time VM access page in Azure Security Center?

The Unsupported tab contains VMs without JIT enabled and which don't support the feature. Your VM might be in this tab for the following reasons:

• Missing network security group (NSG) - JIT requires an NSG to be configured
• Classic VM - JIT supports VMs that are deployed through Azure Resource Manager, not 'classic deployment'.
• Other - Your VM might be in this tab if the JIT solution is disabled in the security policy of the subscription or the resource group.