It's supposed to be TextBox (it's case-sensitive).
EDIT ==================
The green intellisense underline is merely a warning. Try doing this:
<asp:TextBox blah blah ></asp:TextBox>
Sometimes it clears up weird warnings.
Comments:
0) giving a textbox an
ID
of "DatePicker" is confusing. Change your ID to something more appropriate.
1)
Enabled
is true by default, you don't need to specify it.
2) Change your CSSClass name to something that *isn't* "textbox". Call it "textboxStyle" or something like that.