Missing closing }:
@{
var txt = "";
if (DateTime.Now.Hour < 11)
{ txt = "Good Morning"; }
else if (DateTime.Now.Hour > 11 && DateTime.Now.Hour < 16)
{ txt = "Good Afternoon"; }
else
{ txt = "Good Evening"; }
}
<p id="myp" style="text-align:center;color:greenyellow;font-size:large">@txt</p>