Umbraco - First run - connection string failed

The connection string is wrong, trust me :)

Instead of DataSource=. try DataSource=.\sqlexpress

Also ensure that the initial catalog=UmbracoTest is actually the correct name of the database.

Finally, since you are using an Umbraco 6.x version, the log files will be at ~App_Data/Logs but in this case they will just tell you much the same as the install screen, that an SqlException has occurred.

Edit:

Looking at the Umbraco source, the error you see is raised by the following code:

if (_configured == false || 
    (string.IsNullOrEmpty(_connectionString) ||
    string.IsNullOrEmpty(ProviderName)))
{
    return new Result
                {
                    Message =
                        "Database configuration is invalid. Please check
                         that the entered database exists and that the provided 
                         username and password has write access to the
                         database.",
                    Success = false,
                    Percentage = "10"
                };
}

So my assumption is that you are missing the Provider from your connection string. In your web.config file your connection should look like this:

I had the same issue. I fixed it by adding the IIS_IUSRS user to the Umbraco site directory with "Modify" rights as the setup process is trying to write to the web.config with your connection string. Hope this helps.