Need help with twig test template
It looks like you are messing around with your naming.

Since your module is called test_twig_template you must use that name for hooks and namespaces etc.

Fix these things and it should work.


  
  Rename your hook_theme() from test_twig_theme to test_twig_template_theme.
  Call your route test_twig_template.theme.
  Your controller definition in your route must look like this, _controller: \Drupal\test_twig_template\Controller\TestTwigThemeController::content.
  The namespace in your controller must be namespace Drupal\test_twig_template\Controller;.
  Rename test_twig_template/src/Controller/TestTwigController.php to test_twig_template/src/Controller/TestTwigThemeController.php (the file name must match the class name).
  


That should be all, since your module is called test_twig_template it's important to use that name in all namespaces for your code to work properly :)

And then as @Hudri mentions your template shall only contain:

&lt;p&gt;Test twig template!&lt;/p&gt;
&lt;p&gt;test_var: {{ test_var }}&lt;/p&gt;


Head and body tags are provided by a general template in your theme.