Can't print pdf : Notice: Undefined index: thead in /vendor/fooman/tcpdf/tcpdf.php
If you are getting a PHP Notice message when trying to generate any pdf invoice:
Notice: Undefined index: thead in /vendor/fooman/tcpdf/tcpdf.php
it usually happens after adding custom text that includes some html tags with a small error. Please note that the pdf conversion process is not as forgiving as a browser and needs 100% correct html to function.
Here are some examples with incorrect html
<p>Paragraph with only an opening tag <br> there is no br tag, it's officially <br/> etc.
Stores > Configuration > Sales > PDF Print-outs