If you did a fresh installation on a new Ubuntu (~18.04) version, you may find that it is not possible to log in to MySQL with its root user, unless you do so as Ubuntu’s root user. Here’s how to fix it:

1. Log in to MySQL using Ubuntu’s root user.: sudo mysql.
2. Delete current root@localhost account.: DROP USER 'root'@'localhost';.
3. Recreate user.: CREATE USER 'root'@'%' IDENTIFIED BY 'root';.
4. Grant full permissions.: GRANT ALL PRIVILEGES ON *.* TO 'root'@'%' WITH GRANT OPTION;.
5. Flush privileges.: FLUSH PRIVILEGES;.
6. Exit MySQL.: exit;.
7. Log in to MySQL without using Ubuntu’s root user. When prompted, enter the password you assigned above.: mysql -u root -p.

Reference: https://askubuntu.com/a/784347/100470.