After installing mysql 5.6 CLI client, I opened the application it ask for a password that I don't know. Can someone assist?
Depending which 5.6 package you installed it may not have a password. Try 'mysql -u root' to see if you really do not have a password. Or you will have to refer to how to recover the root password in the documentation.
By the way MySQL 5.6 is a VERY OLD release and will not be support for much longer and you SERIOUSLY need to consider upgrading to at least 5.7 if not 8.0