It's the space in your name.
Try this:
DV.RowFilter = String.Format("[User name] Like '%{0}%'", TextBox6.Text)
BTW: Do yourself a favour, and stop using Visual Studio default names for everything - you may remember that "TextBox6" is the user name today, but when you have to modify it in three weeks time, will you then? Use descriptive names - "tbUserName" for example - and your code becomes easier to read, more self documenting, easier to maintain - and surprisingly quicker to code because Intellisense can get to to "tbUserName" in three keystrokes, where "TextBox6" takes thinking about and 8 keystrokes...