I have installed nginx on Raspberry Pi running Raspbian Stretch and installed WordPress
In the process I ran the following

sudo apt install mysql-server
sudo apt install php-mysql

I setup MariaDB and set up WordPress Database with

sudo mysql_secure_installation
sudo mysql -uroot -p
create database wordpress;
GRANT ALL PRIVILEGES ON wordpress.* TO 'root'@'localhost' IDENTIFIED BY 'redacted';

I have done some WordPress configuration, and wanted to backup my WordPress

I attempted to install phpmyadmin (sudo apt install phpmyadmin), but this keeps failing with error ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
There seems to be no option to enter the password for root@localhost

I have tried to interrogate MariaDB which shows:-

MariaDB [(none)]> status
--------------
mysql  Ver 15.1 Distrib 10.1.23-MariaDB, for debian-linux-gnueabihf (armv7l) using readline 5.2

Connection id:      104
Current database:   
Current user:       root@localhost
SSL:            Not in use
Current pager:      stdout
Using outfile:      ''
Using delimiter:    ;
Server:         MariaDB
Server version:     10.1.23-MariaDB-9+deb9u1 Raspbian 9.0
Protocol version:   10
Connection:     Localhost via UNIX socket
Server characterset:    utf8mb4
Db     characterset:    utf8mb4
Client characterset:    utf8mb4
Conn.  characterset:    utf8mb4
UNIX socket:        /var/run/mysqld/mysqld.sock
Uptime:         18 hours 43 min 19 sec

MariaDB [(none)]> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mysql              |
| performance_schema |
| wordpress          |
+--------------------+

So no phpmyadmin database seems to have been created.

Can anyone help me install phpmyadmin ?

Would manually creating database phpmyadmin help?

EDIT

Based on the answer by @Kramer this seemed to be a known problem with Debian (I thought Debian packages were meant to be checked).

I have now performed the following steps (file locations differed from suggested)

cd /usr/share/phpmyadmin/sql/
create database phpmyadmin;

mysql -u root -p < create_tables.sql

    create database phpmyadmin;
    grant all privileges on phpmyadmin.* to phpmyadmin@localhost identified by "redacted";

    Grant USAGE ON mysql.* TO phpmyadmin@localhost;
    GRANT SELECT ON mysql.db TO phpmyadmin@localhost;
    GRANT SELECT (Host, Db, User, Table_name, Table_priv, Column_priv) ON mysql.tables_priv TO phpmyadmin@localhost;
    GRANT SELECT (Host, User, Select_priv, Insert_priv, Update_priv, Delete_priv, Create_priv, Drop_priv, Reload_priv, Shutdown_priv, Process_priv, File_priv, Grant_priv, References_priv, Index_priv, Alter_priv, Show_db_priv, Super_priv, Create_tmp_table_priv, Lock_tables_priv, Execute_priv, Repl_slave_priv, Repl_client_priv) ON mysql.user TO phpmyadmin@localhost;
    GRANT SELECT, INSERT, UPDATE, DELETE ON phpmyadmin.* TO  phpmyadmin@localhost;

I configured nginx by adding the following to /etc/nginx/sites-available/default as per https://www.linuxbabe.com/linux-server/install-phpmyadmin-nginx-ubuntu-16-04

location /phpmyadmin {
  root /usr/share/;
  index index.php;
  try_files $uri $uri/ =404;

  location ~ ^/phpmyadmin/(doc|sql|setup)/ {
    deny all;
  }

  location ~ /phpmyadmin/(.+\.php)$ {
    fastcgi_pass unix:/run/php/php7.0-fpm.sock;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    include fastcgi_params;
    include snippets/fastcgi-php.conf;
  }
 }

Navigating to your-domain.com/phpmyadmin/ and logging in as phpmyadmin showed phpMyAdmin.
I thought success!, however on navigating to Databases I received the following errors:-

mysqli_real_connect(): (HY000/1045): Access denied for user 'root'@'localhost' (using password: YES)  
Connection for controluser as defined in your configuration failed.

It seems the earlier attempt had created/modified files in /etc/phpmyadmin/ which is trying to use the missing password (as per the installation log)

Creating config file /etc/phpmyadmin/config-db.php with new version
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO).
unable to connect to mysql server.
error encountered creating user:
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
dbconfig-common: phpmyadmin configure: trying again.
Determining localhost credentials from /etc/mysql/debian.cnf: succeeded.
dbconfig-common: writing config to /etc/dbconfig-common/phpmyadmin.conf
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO).
unable to connect to mysql server.
error encountered creating user:
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
dbconfig-common: phpmyadmin configure: trying again.
Determining localhost credentials from /etc/mysql/debian.cnf: succeeded.
dbconfig-common: writing config to /etc/dbconfig-common/phpmyadmin.conf
Replacing config file /etc/dbconfig-common/phpmyadmin.conf with new version
Replacing config file /etc/phpmyadmin/config-db.php with new version
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO).
unable to connect to mysql server.
error encountered creating user:
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)