Hello Hugo,
This version of sellacious is not tested completely on Joomla 3.9.x yet.
However, here is the patch for this error.
Please go to
../sellacious/includes/framework.php and find
require_once JPATH_LIBRARIES . '/import.legacy.php';
then at line no. 17 add this patch.
require_once JPATH_PLATFORM . '/loader.php';
JLoader::registerNamespace('Sellacious', JPATH_LIBRARIES . '/sellacious/objects');
JLoader::registerAlias('JToolbarHelper', 'Sellacious\Toolbar\ToolbarHelper');
For more info see the attached screenshot.
This issue will resolve after this.