Did you use the actual Kibana system user? (username "kibana_system" or "kibana" [deprecated])
It might sound counterintuitive, but this is the account that the Kibana server itself uses to authenticate with Elasticsearch. It intentionally has very limited permissions, you cannot log into Kibana with that account.
Edit: Further information can be found in the docs: Configure security in Kibana | Kibana Guide [8.11] | Elastic
This must be a user who has been assigned Kibana privileges. Kibana server credentials should only be used internally by the Kibana server.
If that's the case, try logging in with the "elastic" superuser so you can make yourself a regular account to use in the future =)