Hello @Joshua !
Hope you are having a great day!
Thank you for asking a Question! We are Glad to Assist you!
You can't add an existing VM to an availability set after it's created.
VMs must be created within the availability set to make sure they're correctly distributed across the hardware.
A VM can only be added to an availability set when it is created. To change the availability set, you need to delete and then recreate the virtual machine.
Please refer to the below documents for more information :-
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/tutorial-availability-sets
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/change-availability-set
Let me know if the above solution resolved your issue!
Regards,
Tasadduq Burney
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