Welcome to Android Central! I moved this from the Android Apps forum to the Galaxy S7 forum since the former is intended for developers to promote their apps.
The "application manager" is the Apps menu in the system settings. Swipe down the notification panel, tap the gear icon, and then look in the system settings for the Apps section.
What's the problem with sending text messages? Are you talking about SMS, or some other messaging platform? Are you getting an error message? Do calls work normally?