Hi Dan, here are a couple of things to try.
Hi Dan,
I just created a key vault, tried to add a new cert and of course it fails as expected.
Then I assigned myself the IAM roles: Keyvault admin and cert officer roles - just as you said.
Then I refreshed my browser in order to update my token/roles. (no logout needed)
And now I was successful in creating a new cert.
Is it possible you simply need to refresh your access token? or did someone change the role assignment method under the key vault settings? or are you using PIM?
Good luck.