mPDF temporary directory is not writable by the webserver user

You can resolve this issue by following the steps below

Update the permissions of the following directory {magento_root}/vendor/mpdf/mpdf/tmp to allow the web server write access

Using SSH

You can do this by logging onto your server using SSH and navigating to your magento root folder

Run the following command: chmod -Rf 777 ./vendor/mpdf/mpdf/tmp

Using FTP software

Open your FTP software and navigate to the following directory on your server: {magento_root}/vendor/mpdf/mpdf

Right click the tmp folder and update permissions to write access for the web server user

Clear the full page cache

After changing the directory permissions, please don't forget to clear the Magento cache.