why the logout button doesn't appear in kibana 7.5. Please anyone help ..

It should still be there assuming you have security enabled and you're logged in. Have you tried logging out, clearing browser cache, and logging back in?
© 2020. All Rights Reserved - Elasticsearch
Apache, Apache Lucene, Apache Hadoop, Hadoop, HDFS and the yellow elephant logo are trademarks of the Apache Software Foundation in the United States and/or other countries.