The html pages must have only one tag <title>, <body> <head> ... (see here or here).
Your custom code Your code redefines these tags.
If you really want to insert that code, you should edit it by moving the CSS part in the appropriate section (Appearance->custom CSS), so paste
Code:
@import url(https://www.w3schools.com/w3css/4/w3.css);
.
Then change your html code in:
HTML Code:
<div class="w3-container">
<div class="w3-bar-block" style="width:100%">
<a href="http://ow.ly/i0uO30l6r5G" class="w3-bar-item w3-right-align w3-hover-purple">مشاوره آیلتس و تافل</a>
<a href="http://ow.ly/wcxo30l6qXN" class="w3-bar-item w3-right-align w3-hover-indigo">ترجمه فارسی به انگلیسی</a>
<a href="http://ow.ly/tiVF30l6r9g" class="w3-bar-item w3-right-align w3-hover-blue">طراحی وب*سایت انگلیسی</a>
<a href="http://ow.ly/V0wA30l6raj" class="w3-bar-item w3-right-align w3-hover-green">راهنمایی در تسلط بر کتاب ۵۰۴</a>
</div>
</div>
Anyway, for your initial problem (for white space) you could solve by adding below the following CSS code (Appearance-> custom CSS):
Code:
.header-image{
margin-bottom: 0px;
}
Bye!