Hi Hyling,
In 5.1.1, reset admin password needs a flag -c/–cluster .
In your command, remove double/single quote around your host and you don’t need to put port number if you use default port (8091). It will reset you password with new password
[root@s44015 ~]# /opt/couchbase/bin/couchbase-cli reset-admin-password -c localhost --new-password password1
SUCCESS: Administrator password changed