For running J!5.1 you must https://fabrikar.com/forums/index.php?wiki/update-from-github/ or include the new file manually https://fabrikar.com/forums/index.php?threads/joomla-5-1-and-fabrik-cannot-find-files-error.54473/post-285151 See also Announcements
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Is that a part of the VirtuMart core, or a plugin? I have VM installed, but don't see an option for packing slips.Fabrik V
3.8.1
Regarding Virtuemart's PDF, I'm not sure. There was one functioning before i installed Fabrik, Im not sure if its VM or Joomla
Did you try to re-install this tcpdf extension?I assume they are "sharing" the same PdfDocument.php file with Fabrik (can't test because it's paid).
So which ever is installed latest will override the other one (I can remember such an issue with Phoca PDF).
You can try to re-install vm-invoice-generator (but this will then break Fabrik PDF...)