In some versions of Magento 2 we are facing issues like this where error occurs in all sent mails.

I tried so many solutions related .less file also tried adding “pelago/emogrifier”: “1.0.0 as 0.1.1” in our root composer.json

But it never worked out. If you wish, you can try it and check. In case, if you get negative signal, try below proven method which helped me.

Go to Admin Marketing Menu – Email Templates and add new custom template then click on Load template button and load Magento_Email – Header

After load that template content remove below line and save it,

This one works in most of the cases. It worked for me in Magento 2.3 versions, Hope this will help you to solve the error.