Unable to find template in Symfony 3

This helped me in my case: @Core/index.html.twig

try to change this:

return $this->render('CoreBundle:index.html.twig');

to this:

return $this->render('CoreBundle::index.html.twig');

The difference is to use :: instead of :

According to the Symfony 3 Docs it's better to use slashes and to not use "Bundle" word.

So, we have as example:

return $this->render('@BloggerBlog/Default/index.html.twig');