Can you please confirm that you have been assigned the role as "Owner" for the subscription you are using?
We have seen this issue occur when the user have 'Service Administrator' role instead of 'Owner' role. Unfortunately Service Administrator role role does not support changing permission model as mentioned in the below document:
https://video2.skills-academy.com/en-us/azure/key-vault/general/rbac-guide?tabs=azure-cli
Let me know and if this is not the reason, I can further investigate. Thanks.