Thanks for posting your question in the Microsoft Q&A forum.
To resolve it, check your permissions in Access Control or Access Policies, and verify that any managed identities have “Set” rights. Testing secret creation via Azure CLI can reveal more details. If permissions cannot be adjusted due to subscription restrictions, upgrading to a standard or trial plan may be necessary.
Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful