Blogger Templates must contain following blocks
b:skin section.This is nothing but css style sheet.You can also link css page.But you must have this sectionb:section and b:widget.This is a section which is used for adding elements like navbar,adsense,sidebar.Atlease template must have one b:section
b:widget.Elements are called as widgets.It must be placed inside b:section
Simple Blogger Template code
<html><head>
<link href='https://www.w3schools.com/w3css/4/w3.css' rel='stylesheet'/>
<b:skin></b:skin>
</head>
<body>
<b:section class='header' id='header' maxwidgets='1' showaddelement='no'/>
<b:section class='sidebar' id='sidebar' maxwidgets='' showaddelement='yes'/>
<b:section class='main' id='main' name='Main' showaddelement='no'>
<b:widget id='Blog1' locked='true' title='Blog Posts' type='Blog'>
</b:widget>
</b:section>
<b:section class='footer' id='footer' showaddelement='no'/>
</body>
</html>