This is the code for parts/sidebar.html—

<!-- wp:group {"className":"sidebar"} -->
<div class="wp-block-group sidebar"></div>
<!-- /wp:group -->

In the backend here are flywheel installations it appears as this—

Intended was to get a fetaure to add blocks, but could not.

This is index.html in templates/index.html—

<!-- wp:group {"className":"wrapper inset-layout","layout":{"type":"default"}} -->
<div class="wp-block-group wrapper inset-layout">
    <!-- wp:group {"className":"division","layout":{"type":"default"}} -->
    <div class="wp-block-group division">
        <!-- wp:template-part {"slug":"sidebar"} /-->

        <!-- wp:group {"className":"main","layout":{"type":"default"}} -->
        <div class="wp-block-group main">
            <!-- wp:group {"className":"stack","layout":{"type":"default"}} -->
            <div class="wp-block-group stack">
                <!-- wp:group {"className":"primary-content","layout":{"type":"default"}} -->
                <div class="wp-block-group primary-content">
                    <!-- wp:heading {"level":1} -->
                    <h1><!-- wp:post-title {"isLink":true} /--></h1>
                    <!-- /wp:heading -->
                    <!-- wp:post-content /-->
                </div>
                <!-- /wp:group -->
                <!-- wp:eternaltwentyfifteen/contentmetafooter /-->
            </div>
            <!-- /wp:group -->
        </div>
        <!-- /wp:group -->
    </div>
    <!-- /wp:group -->
</div>
<!-- /wp:group -->

Whats the issue where am I wrong?

File/Folder Structure:

This is Folder/File Structure—

eternaltwentyfifteen/
├── assets/
│   ├── css/
│   │   ├── layout.css
│   │   ├── mobile.css
│   │   ├── post.css
│   │   └── style.css
│   └── js/
│       └── custom.js
├── build/
│   ├── contentFooter.asset.php
│   ├── contentFooter.js
├── our-blocks/
│   └── contentFooter.js
├── parts/
│   ├── main.html
│   └── sidebar.html
├── templates/
│   └── index.html
├── functions.php
├── index.php
├── package-lock.json
├── package.json
├── style.css
└── theme.json