The WatermarkSheetsFolios association is not defined on WatermarkSheets. 📋

Valid associations are: Facts InvalidArgumentException
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            $message "The `{$name}` association is not defined on `{$this->getAlias()}`.";
            if ($assocations) {
                $message .= "\nValid associations are: " implode(', '$assocations);
            }
            throw new InvalidArgumentException($message);
        }
        return $association;
    }

If you want to customize this error message, create templates/Error/error500.php