How to set links correctly in the toolbar?

I have a question about links in the toolbar. I would like to use the following structure:

content
├── handbook
│   └── _index.md
├── index.md
├── products
│   └── _index.md

Unfortunately, the links in the toolbar do not work properly.

I have the following code in the head.html:

<header class="mdc-toolbar mdc-toolbar--fixed mdc-toolbar-header mdc-theme--priamry-bg">
      <div class="mdc-toolbar__row">
        <section class="mdc-toolbar__section toolbar__section--align-start">
          <button class="header-menu material-icons mdc-toolbar__menu-icon">menu</button>
          <nav id="header-tab-bar" class="mdc-tab-bar mdc-tab-bar--indicator-secondary">
            <a class="mdc-tab mdc-tab--active" href='{{ relref . "index.md" }}'>Home</a>
            <a class="mdc-tab" href='{{ relref . "handbook/_index.md" }}'>Handbook</a>
            <a class="mdc-tab" href='{{ relref . "projects/_index.md" }}'>Projects</a>
            <span class="mdc-tab-bar__indicator"></span>
          </nav>
        </section>
        <section class="mdc-toolbar__section mdc-toolbar__section--shrink-to-fit mdc-toolbar__section--align-end">
          <span class="mdc-toolbar__title">Developer</span>
          <img src="{{ "images/logo.jpg" | absURL }}" alt="" class="mdc-toolbar__logo">
      </div>
    </header>

Here the error message.

Started building sites …
ERROR 2017/11/27 16:05:54 Error while rendering “page”: template: theme/_default/single.html:1:3: executing “theme/_default/single.html” at <partial "header.html…>: error calling partial: template: theme/part»

ERROR 2017/11/27 16:05:54 Error while rendering “section”: template: theme/_default/list.html:1:3: executing “theme/_default/list.html” at <partial "header.html…>: error calling partial: template: theme/parti»

ERROR 2017/11/27 16:05:54 Error while rendering “taxonomy”: template: theme/_default/list.html:1:3: executing “theme/_default/list.html” at <partial "header.html…>: error calling partial: template: theme/part»

ERROR 2017/11/27 16:05:54 Error while rendering “taxonomyTerm”: template: theme/_default/terms.html:1:3: executing “theme/_default/terms.html” at <partial "header.html…>: error calling partial: template: them»

ERROR 2017/11/27 16:05:54 Error while rendering “home”: template: theme/index.html:1:3: executing “theme/index.html” at <partial "header.html…>: error calling partial: template: theme/partials/header.html:10:»

Here is the repo: https://gitlab.com/joklein/hugo

Thank you for the help.

You probably want to link to those sections using a different method.