There are a few posts noting this error message, without a solution.
In case somebody stumbles upon it : This message may be very misleading, in case you did not activate xpack security
First, check your service logs:
journalctl -u kibana.service
if you find the following message, there you go`
Kibana security must be enabled to use Fleet
configure x-pack to true in elasticsearc.yml, restart both elasticsearch and kibana
you will need then to secure the access
You will need the following commands on terminal
/usr/share/bin/elasticsearch-create-enrollment-token --scope kibana
/usr/share/kibana/bin/kibana-verification-code
/usr/share/elasticsearch/bin/ elasticsearch-reset-password -u elastic -i
hope this helps someone
you're welcome