I faced the same issue once when I upgraded Elasticsearch from v6 to v7.
Deleting .kibana* indexes fixed the problem:
curl –request DELETE ‘https://elastic-search-host:9200/.kibana*’
Probably not the solution for this question
In my case the version from kibana and elasticsearch were not compatible
How i was using docker, i just recreated both but using the same version (7.5.1)
https://www.elastic.co/support/matrix#matrix_compatibility