Hello Juan,
Can I know which version of sellacious and Joomla you're using? The issue with TCPDF fixed in the sellacious release 1.7.1.
The latest release of sellacious 1.7.3 will be out this week. Update the sellacious to the latest version and this issue will be fixed.
If you're still having this issue after the update, let us know.
I hope this helps.
Thank you