Have you followed all the instructions in the Azure AD portion of the OIDC provider supplemental documentation?
There have been a handful of others that had a similar issue:
Azure OIDC Group claim not found, now working but why?!?! and related Vault Azure AD with OIDC - claim "upn" not found in token
If those don’t help, then provide some more of your configuration details so we can better help troubleshoot (redact any sensitive info, of course).