Some apps require the device admin permission to perform some actions. Some of these actions are the factory reset, screen lock, etc. If an app is installed into the external storage (SD card), you can see the following warning into the logcat, when it tries to get such a permission:

W DeviceAdminAdd: Exception trying to activate admin
  ComponentInfo{de.nulide.findmydevice/de.nulide.findmydevice.receiver.DeviceAdminReceiver}
W DeviceAdminAdd: java.lang.IllegalArgumentException: Only apps in internal storage can be active
  admin: ComponentInfo{de.nulide.findmydevice/de.nulide.findmydevice.receiver.DeviceAdminReceiver}

If an app is installed into the external storage it will not be displayed into Settings ⇒ Apps & notifications ⇒ Special app access ⇒ Device admin apps. To move the app into the internal storage (device memory) you can go through Settings ⇒ Storage ⇒ SD card ⇒ Apps. Then tap on the app and verify the Storage used, eventually tap the CHANGE button.