Delete the main.html file and setup the mainlayout.html file like this below. You don’t need to declare a body tag. You can setup a base layout template to render to the lowest level if you need, and have the content/mainlayout inside that also
<head>
<title>Taskmanager</title>
</head>
<template name="mainlayout">
{{> navBar}}
<main>
{{> Template.dynamic template=main}}
</main>
</template>
// You can render dynamically to different areas in the base layout if needed, not only the main content
BlazeLayout.render('layoutAppBase', {
sidebar: 'mainSidebar',
sideWindow: 'appSideWindowBase',
sideNav: 'appSideNav',
content: 'mainContentExample'
})