In some circumstance you may get the following error when trying to give some permissions to an app:

Restricted Setting
For your security, this setting is currently unavailable.

This may include e.g. the notifications or the accessibility permission.

To remove such restriction do the following:

• Settings ⇒ Apps & Notifications, search the app and tap it.
• Tap on three dots.
• Choose “Allow restricted settings”.

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.