Get error Access denied for user 'root'@'localhost' (using password: NO) when setting Wordpress on amazon ec2

Question

I have a problem when I setting up wordpress on amazon ec2, but I'm getting an issue about 'Access denied'.

[root@ip-172-31-25-142 ec2-user]# mysql -u root
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)

[root@ip-172-31-25-142 ec2-user]# mysqladmin -u root -p password 6k8zJHLrmWB8bmwV

Enter password:

mysqladmin: connect to server at 'localhost' failed
error: 'Access denied for user 'root'@'localhost' (using password: YES)'

Anyone can help me! Many thank!!!!