Hi Yousuf,
Thanks you for your message, I understand that you no longer receive notifications for errors occuring with you project.
To answer your question, App Engine Flex comes by default with Stackdriver Error Reporting is enabled and there is no additional step to take for this.
That being said you could be experiencing another issue that you can report this using the Public Issue tracker via this link where our colleague will assist you as Google Group forum is intended for general discussions on the App Engine platform, not specific technical issues. .