I am getting the notification: Disable harmful app. On disabling it, I am still getting back the notification. What do I do?
You can disable the security popup by Settings -> Google -> Security -> (Play Protect) And uncheck “Scan Device for Security threats”

If there are more details under that tab, then comment below.

For safer app installation, you can disable Unknown Sources in Settings -> Security -> Disable "Allow installation of apps from unknown source"

For safer browsing, you can install firefox for Android, and add ublock origin as an adblocker extension when browsing the web.