Fatal error: Uncaught Twig\Error\LoaderError in OpenCart

Twig\Error\LoaderError error in OpenCart occurs when the system cannot load the Twig template due to a missing file, invalid paths, or version incompatibility.

Major causes of the error

1. Incorrect paths to Twig files in the code.
2. Missing Twig template files
3. Incompatibility of OpenCart and Twig versions. Especially after updating OpenCart to 3.0.3.6 and higher.

Solution for different OpenCart versions

OpenCart to 3.0.3.6

1. Update the modifier cache and the template engine cache on the main admin page by clicking on the gear button.
2. Editing the template loader: In the file system/library/template/twig.php replace: 

$loader = new \Twig_Loader_Array(array($filename . '.twig' => $code));

to

$loader1 = new \Twig_Loader_Array(array($filename . '.twig' => $code));
$loader2 = new \Twig_Loader_Filesystem(array(DIR_TEMPLATE)); // to find further includes
$loader = new \Twig_Loader_Chain(array($loader1, $loader2));

OpenCart 3.0.3.6 and higher

$loader = new \Twig\Loader\ArrayLoader(array($filename . '.twig' => $code));

to

$loader1 = new \Twig\Loader\ArrayLoader(array($filename . '.twig' => $code));
$loader2 = new \Twig\Loader\FilesystemLoader(array(DIR_TEMPLATE)); // to find further includes
$loader = new \Twig\Loader\ChainLoader(array($loader1, $loader2));

Following these recommendations will help prevent the error from occurring again and ensure stable operation of the store.