Problem with Android Device Manager.
Android Device Manager is a tool to locate lost Android device or to ring or to erase data in the Android. It is mainly to remotely control your phone.
However if you don’t see your device make sure that you’re signed in to your Google Account, your location settings are on, and that you’ve turned on Android Device Manager in Google Settings.
If you still do not find your device then :
1) Unhide device on Google Play
2) Use same Google account.
Unhide device on Google Play
If a device is hidden in Google Play then it will not appear in Android Device Manager. So you have to check that your device is not hidden in Google Play. To check this just follow below steps:
Sign in with your google account
In the “Visibility” column, select a device.
This will make your device visible in Google Play.
Use same Google account.
While using Android Device Manager you must be signed in to google account in the device and your system.