javaScript Runtime error:'mainmanu' is undefined pls help

Question

My Code Is:- 

 

using Visual studio

 

<script src="Scripts/jquery-3.3.1.min.js" type="text/javascript"></script>

<script type="text/javascript">

function mainmanu() {

$(" #nav ul ").css({ display: "none" }); 

$(" #nav li").hover(function () {

$(this).find('ul:first').css({ visibility: "visible", display: "none" }).show(400);

}

, function () {

$(this).find('ul:first').css({ visibility: "hidden" });

});

}

$(document).ready(function () {

mainmenu();

});

</script>