Hello,
as the error message claims, the reason for it is that the class
TCPDF (that is responsible for the PDF generation) is already defined. Probably this is done by some other component or module/plugin that runs before VM Invoice.
A hotfix for this would be editing the file
administrator/com_vminvoice/helpers/invoicepdf.php.
and wrapping the lines:
require_once (dirname(__FILE__) . '/../tcpdf/tcpdf.php');
require_once (dirname(__FILE__) . '/../tcpdf/config/tcpdf_config.php');
with a condition:
if (!class_exists('TCPDF')) {
require_once (dirname(__FILE__) . '/../tcpdf/tcpdf.php');
require_once (dirname(__FILE__) . '/../tcpdf/config/tcpdf_config.php');
}
Just please note, that the TCPDF library that is loaded at that time MAY be different version that is compatible with VM Invoice. In such case, it would be neccessary to identify, where in your site's code it is preloaded and prevent it.