Unable to see Availability Set option when creating VM in Azure

Why is the option missing?

No documented rule for when the portal shows or hides it. But it's offering you Availability Zone, which suggests your region does have zones now. Microsoft has been adding them over time, so a list you read may predate that. Worth confirming which region is selected.

For a single VM you pick either a zone or a set, so selecting a zone hides the set option.

If no zone is selected and it's still missing, check whether the region supports aligned sets:

az vm list-skus --resource-type availabilitySets \
  --query "[?name=='Aligned'].{Location:locationInfo[0].location, MaximumFaultDomainCount:capabilities[0].value}" \
  -o table

Not in the output, that's your answer. In the output, it's the VM config.

Which regions support Availability Sets?

There isn't a list. They aren't region-gated the way zones are. The command above gives you the fault domain count per region, which is the thing that actually varies.

Does VM size or image type affect it?

Managed disks are required:

Only VMs with managed disks can be created in a managed availability set.

Past that nothing about size or image is documented, which is why the SKU check beats guessing.

Also worth knowing

You can't add an existing VM to a set, it's create-time only. And the portal hiding the option doesn't block you, since ARM, CLI and PowerShell all still take the availability set parameter.

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In short, this aligns with the recommended approach to providing redundancy within a single Azure datacenter that relies on VMSS in flexible orchestration mode (more at https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/overview ). So, you might want to consider using VMSS in flexible orchestration mode instead.

However this is purely a change in the UI. It does NOT mean that availability sets are not available or supported. The availability sets remain fully supported in all Azure regions.

You can still access the previous interface by selecting "Click here to access the previous experience." in the notification that appears at the top of the Basic tab when creating a VM in the Azure portal. Just look for the wording:

This is a preview of a new Create-VM experience. Features are limited in this experience and all feedback is appreciated. Click here to access the previous experience.

Obviously you can also create/use availability sets via PowerShell, CLI, REST API, or templates.

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hth

Marcin