Magento2 – Invalid ExtensionInterface for Nonexistent Class Interface with setup:di:compile

Take a look here:

Magento\Framework\ObjectManager\ObjectManager->get() at /opt/bitnami/apps/magento/htdocs/app/code/J2t/Rewardpoints/Observer/ProcessSaveModel.php:21

Looks like this module is trying to grab the Object Manager which might be triggering the error if that area code is not set. In my comment, i linked to a solution i found to this error for an extension i was using. Might help you out here as the call to the Object Manager is happening in an Observer.

Here is that link again: Magento 2 "Area code not set" warning in 3rd party module

That's an interesting issue and I'm not entirely sure why is this happening.

What I can suggest is debugging directly in the _findMissingClasses method from Magento/Setup/Module/Di/Code/Scanner/PhpScanner that's where the error is coming from:

protected function _findMissingClasses($file, $classReflection, $methodName, $entityType)
{
    $missingClasses = [];
    if ($classReflection->hasMethod($methodName)) {
        $constructor = $classReflection->getMethod($methodName);
        $parameters = $constructor->getParameters();
        /** @var $parameter \ReflectionParameter */
        foreach ($parameters as $parameter) {
            preg_match('/\[\s\<\w+?>\s([\w\\\\]+)/s', $parameter->__toString(), $matches);
            if (isset($matches[1]) && substr($matches[1], -strlen($entityType)) == $entityType) {
                $missingClassName = $matches[1];
                try {
                    if (class_exists($missingClassName)) {
                        continue;
                    }
                } catch (\RuntimeException $e) {
                }
                $sourceClassName = $this->getSourceClassName($missingClassName, $entityType);
                if (!class_exists($sourceClassName) && !interface_exists($sourceClassName)) {
                    $this->_log->add(
                        Log::CONFIGURATION_ERROR,
                        $missingClassName,
                        "Invalid {$entityType} for nonexistent class {$sourceClassName} in file {$file}"
                    );
                    continue;
                }
                $missingClasses[] = $missingClassName;
            }
        }
    }
    return $missingClasses;
}

That method is called from the _fetchFactories method which also logs the same error so you need to find out, where the error is coming from exactly and from there you should be able to start debugging:

protected function _fetchFactories($reflectionClass, $file)
{
    $factorySuffix = '\\'.ucfirst(FactoryGenerator::ENTITY_TYPE);
    $absentFactories = $this->_findMissingClasses(
        $file,
        $reflectionClass,
        '__construct',
        ucfirst(FactoryGenerator::ENTITY_TYPE)
    );
    foreach ($absentFactories as $key => $absentFactory) {
        if (substr($absentFactory, -strlen($factorySuffix)) == $factorySuffix) {
            $entityName = rtrim(substr($absentFactory, 0, -strlen($factorySuffix)), '\\');
            $this->_log->add(
                Log::CONFIGURATION_ERROR,
                $absentFactory,
                'Invalid Factory declaration for class ' . $entityName . ' in file ' . $file
            );
            unset($absentFactories[$key]);
        }
    }
    return $absentFactories;
}