I saw a similar issue before that may be related to your problem. When we use the "Azure Appservice certificate" the overview page shows the option to "Configure required KeyVault store." Then it will direct to the page "Store, Verify, Assign." Once the KeyVault is imported this operation should be succeeded.
The likely cause of your problem is that the Azure KeyVault with RBAC support model doesn't support the Key Vault RBAC permission model and it is necessary to use Azure Key Vault with access policies method. You need to either change the access model to "access policies" or (if you don't have permission to change the access model) create a new Key Vault with access policy as the method.
Our documentation mentions this limitation.
Let me know if this helps and if you still face this issue.
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