It's a completely new site and only genuine plugins installed on it. We tried malware and virus scanning from our end and the hosting provider's side also. We could not found anything!, contacted Microsoft support using the "Report that this site doesn’t contain phishing threats" page. But no use!
So at last, we removed the entire WordPress installation, then re-installed the WordPress on this domain. Then also did malware and virus scanning, Contacted Microsft defender through "Report that this site doesn’t contain phishing threats" page. But still, it shows this warning. We hired a cybersecurity professional to fix it, he also can't fix it.
Now, I believe only Microsoft people can fix it!.
We created this portal as an education platform for entrepreneurs. It's one of my dream projects! stuck due to this Warning.