This problem may be caused by the following changes.
Elasticsearch 6.7.0 checks that there are no leftover geoip database files in the plugin configuration directory because the new module does not use them. Therefore, remove the ingest-geoip plugin prior to upgrading to 6.7.0 with the --purge option to also delete the old database files:
> ./bin/elasticsearch-plugin remove --purge ingest-geoip
Otherwise you will see the following error message upon startup (assuming /etc/elasticsearch/ingest-geoip as the plugin configuration directory):
expected database [GeoLite2-ASN.mmdb] to not exist in [/etc/elasticsearch/ingest-geoip]
https://www.elastic.co/guide/en/elasticsearch/reference/current/breaking-changes-6.7.html#_literal_ingest_geoip_literal_and_literal_ingest_user_agent_literal_are_no_longer_distributed_as_plugins