If you are 100% sure that the username and password are correct, then the only possibility is that the database or server name are wrong, or that the web server is being blocked from connecting to the database server.
The best way to find out about this is talking to your hosting company to see what they can find out for you.
I am 100% sure the username and password are correct because I don’t have MsSql installed so the username and password are taken from the installation prompt.
You don’t need to have MySQL installed on your PC, but you do need it to be accessible from the hosting server. Just because you can connect from your local PC doesn’t mean that your web server can do the same.
As I said before, you do need to check this with your hosting company as they are the ones that will have the proper ability to look into the settings for your website.
The installation sees that I don’t have MySQL installed and installs it automatically. The installation fails because what is installed during the installation can’t connect with the WordPress installation.
Which installation installs MySQL? WordPress doesn’t do that itself, so I’m guessing that you’re using a 3rd party installer somewhere?
Are you installing on your local PC, on a remote PC, on a web server?
If it’s yur local PC or a remote PC that you have control over, then whatever is installing MySQL will set some password for the root account. You will need to find out what the is from whatever software is performing that installation.
If it’s on a web server, MySQL should already be installed, and you’ll need to ask your hosting company for help.
Have you verified that you are using the correct hostname?
