This error message typically indicates that there is an issue with autoloading the necessary class file for the PDFMergerFacade. Here are some steps you can take to resolve the issue:

1. Check if the PDFMergerFacade class exists in your project’s file structure. If it doesn’t exist, try reinstalling the package or library that provides this class.
2. Ensure that you have imported the class correctly using the use statement at the top of your PHP file.
3. Check your composer.json file to make sure that the package or library that provides the PDFMergerFacade class is listed as a dependency and has been installed correctly. If not, try running composer update to ensure that all dependencies are up to date.
4. If you are using a framework such as Laravel, try running php artisan config:cache or php artisan optimize to clear the cache and regenerate the autoload files.
5. Finally, if none of the above steps work, try running composer dump-autoload to regenerate the autoload files manually.