No, you're not mistaken.
I wonder if there's a problem with how your browser is interacting with Vanguard's web interface. Was the upper right part of the window on the "Vanguard Target Retirement 2050 Fund" page showing you as being logged in when you clicked "buy"? The behavior you're describing sounds like what would happen if it thinks that nobody is signed in.
That shouldn't be an issue though.
Go to your Dashboard on the Vanguard website, open the "Transact" drop-down, and then click on "Buy & Sell."
At this point, you should be on a screen that's asking you for a "ticker symbol." A ticker symbol is just a string of characters used to identify something like a stock or a fund. For example, "MSFT" is the ticker symbol for shares of Microsoft stock. If you go back to the "Vanguard Target Retirement 2050 Fund", you'll see the letters "VFIFX" splashed out huge at the top of the page. "VFIFX" is the ticker symbol for this fund, so that's what you would use if that's the fund that you want. Punch that in, and hit the button for the quote, and that should hopefully get you where you're trying to be.
However, I just looked a bit more thoroughly through the old thread, and you said you'd rather aim for 2060? That would be
this fund, in which case you'd want to use ticker symbol
VTTSX.