I have same issue. certificate is installed into API Management's Certificate tab (from Key Vault). it still gives the error. Below is the error I receive:
One or more fields contain incorrect values:
Error in element 'authentication-certificate' on line 16, column 10: The Certificate with id 'testKVCert' and thumbprint 'XXXXXXXXXXXXXXXXXX' is configured with KeyVault secret 'https://kvtestldb.vault.azure.net/secrets/CERTtest' and cannot be referenced by thumbprint. Please reference it with certificate-id 'testKVCert' in the policy.
How can I fix this issue? Please help!