If you are able to install the module without any errors then you might be missing the call to "external_layout" template which generates the default header. You need to call it just before the page starts: Here is a sample

<template id="report_saleorder_document">

        <t t-call="report.external_layout">

                <div class="page">

                     YOUR REPORT CONTENTS

                </div>

        </t>

</template>