Manuall "TijsVerkoyen\CssToInlineStyles\CssToInlineStyles" install path

While sending an email using the Symfony Mailer Lite module, the following error occurred:
Error: Class "TijsVerkoyen\CssToInlineStyles\CssToInlineStyles" not found.
The required CssToInlineStyles library is missing because it wasn’t installed via Composer, which is normally used to manage dependencies.

Download the Library:
The CssToInlineStyles library was manually downloaded from GitHub.

Place the Library:
The library was placed in the path:
vendor/tijsverkoyen/css-to-inline-styles.

Autoload the Library:
It was manually added to Drupal's autoloader by including it in the autoload.php file.

Clear Cache.