Sometimes when the database does not start it could be something as simple as unnecessary characters in the neo4j.conf file. For example if you add space and/or tab characters to below setting:
The database will fail to start and show you this message:
Caused by: org.neo4j.kernel.lifecycle.LifecycleException: Component 'org.neo4j.server.database.LifecycleManagingDatabase@691eb782' was successfully initialized, but failed to start. Please see the attached cause exception "/var/lib/neo4j/certificates/client_policy/neo4j.key (No such file or directory)".
Make sure to edit the neo4j.conf file with a text file editor and not a word processor to reduce the risk of such issues.