P.S: User = Root, and privileges are ALL YES in 'magento2' DB. M2.2.3 and PHP 7.
You need to make sure you put correct username and password for your user. If they are correct, make sure the user has all privileges to the database.
See this: Error display on Setup upgrade command
Please try to clarify if the user Magento is using is named "root" or if you are running the command as "root" of your system.
bin/magento setup:upgrade is going to take the values shown in
magentoroot/app/etc/env.php inside the database context. I am not sure if it is the same path in windows but the file should have the same name, so look for it.
Then make sure your database user has all privileges(No Grant for obvious reasons) on the database your magento is using(also found in
To Grant all privileges, login to MySql and then :
GRANT ALL PRIVILEGES ON database_magento.* TO 'magento_username'@'x';